12V Link

In Progress

This chapter is incomplete - still being written/defined.

This section addresses the electrical and communications properties of an Open DC Grid (ODG) link implemented as a DC bus with a nominal voltage of 12V. The ODG 12V Link includes a communications capability based on the LIN Bus using a third signal wire besides the two power poles. Communications is required to be provided by all conforming power sources but optional for loads such as appliances.

A bus connection means that multiple devices are electrically connected via wires and will observe roughly the same electrical potential though the actual observed potential may differ between devices due to resistive drops on the wires.

The actual wiring may be in a variety of topological configurations such as:

• point to point - two ports connect with a single link
• star where all drops are connected at a single point
• linear configuration with multiple taps branching from a distribution
• tree-like configuration with multiple branch points fanning out into a web of connections.

All conforming ODG 12V link devices must implement a sleep mode that reduces its current consumption when the supplied voltage drops below a defined threshold to avoid draining battery sources to unhealthy levels. In sleep mode, the communications and management circuits are expected to operate normally. At a still lower voltage device are expected to go into a deeper form of sleep referred to here as disconnect mode that consumes negligible power to protect the power. When normal voltages are restored, devices shall resume normal operation without user intervention.

The ODG 12V Link is defined for interoperability with most existing 12V appliances and power sources. Most existing 12V appliances can be attached to an ODG conforming power source and will operate as expected. In this section, such devices will be referred to as legacy devices. Physical connectivity can be implemented with a passive adapter that matches the legacy device's connector to the connector specified here. Similarly, ODG 12V Link conforming appliances and other loads will operate as expected except for communications when attached to existing 12V power sources such as a 12V battery or automobile auxiliary power connector using a passive adapter.

Devices conforming to the ODG 12V Link allow consumers to attach appliances to power sources and be confident that their appliance will operate as expected but also have additional benefits for consumers and resellers. The ODG 12V Link contains features such as polarity protection that make it goof resistant when attached using ad-hoc wiring. When implemented by appliances, the optional communications capability enables many functions such as power allocation, remote control, management and PAYG. Multiple conforming sources can be attached to a single 12V bus with default or configuration-specific power allocation.

Low voltage DC connectivity has long been challenged by conflicting definitions for the commonly used coaxial connectors, often referred to as barrel connectors. The ODG 12V Link uses such a connector. It attempts to mitigate the risk of conflicting definitions by selecting a connector that is less commonly used except for the specific case of laptop AC power adapters and this standard contains provisions that specifically permit such adapters to be attached to conforming appliances. Such appliances will operate as expected when attached to an AC power adapter so long as the adapter can supply sufficient current and will fail gracefully when the adapter cannot supply enough current.

The ODG 12V Link also provides compatibility to the ISO 16750 standard for automotive electric devices in that devices conforming to the ISO standard will operate when connected to an ODG power source and ODG conforming appliances will operate when connected to an ISO power source. Most of the ISO test requirements also apply to ODG 12V Link conforming devices but some extreme requirements specific to the automotive environment are not required of ODG 12V Link devices.

The ODG 12V Link supports communications between devices attached to the bus through a variation of the LIN Bus used for low-speed communications in automobiles. This communications interface is optional for load devices but required for power sources except for AC adapters. These communications are used to manage a bus with more than one power source, implement priority allocation of power to loads, and for the remote management of appliances or other loads that choose to implement it including PAYG. The communications system includes the concept of a functional bus manager but this function is not restricted to a particular physical device as with a typical LIN Bus and can potentially be shifted between devices as needed when devices are attached or removed from the bus. While capable of general purpose messages, this communication system is intended primarily for bus management and should not be considered a substitute for high speed communications through WiFi or other technologies. Communications details are provided in the following sections.

Port Types

As defined the the architecture section, this standard defines a port as an electrical interface to a device with specified characteristics. The ODG 12V link defines four types of ports:

• Load ports only consume power such as the port on an appliance
• Source ports only supply power
• Bidirectional ports can consume or supply power like a battery
• AC adapter ports are a special kind of source port with reduced functionality

In the simplest configuration, a load port is attached to a source port with a cable. More complex combinations are possible such as a source that powers multiple loads or an arbitrary collection of load, source and bidirectional ports attached to a common bus. AC adapter ports can only attach to a single load port or a bidirectional port with the optional capability to support AC adapters.

Connectors - Plugs and Sockets

The ODG 12V Link is based on a standard coaxial (barrel) connector currently used on many laptop AC power adapters:

• 7.5 mm outside diameter
• 5.0 mm inside diameter
• 3 pole

This connector is wired with the negative pole on the outer surface, the positive pole on the inner surface. The center pin is used for data communications.

Source ports and bidirectional ports always use a female socket. Load ports are recommended to also use a female socket assuming a detachable male plug-to-plug cable but they may optionally use a hardwired cable with a male plug. Use of the female socket on load ports is recommended so they can be powered by existing AC power adapters that come with hardwired male plugs on a cable. AC adapter ports are assumed to always have a male plug on a cable.

Electrical Requirements

This section defines the electrical requirements of the ODG 12V Link which can vary by type of port. The following table of symbols applies to all parts of this section:

Symbol	Value	Description
Vmax_s	15 V	maximum source voltage
Vmax_l	20 V	maximum voltage for loads operating normally
Vmax_a	20 V	maximum AC adapter voltage
Vovp	25 V	minimum overvoltage protection
Vovs	25 V	source disconnect overshoot
Vcond	25 V	load disconnect overshoot
Vmin_s	10.5 V	minimum source voltage
Vmin_l	10 V	minimum voltage for loads operating normally
Vmin_c	9 V	minimum voltage for communications circuits
Vreverse	-25 V	minimum reverse protected voltage
VAC	1 V P-P	minimum AC noise protection
Vdropout	4.5 V	maximum voltage during drop-out protection test
Imax	8 A	maximum current through a connector
Isleep	10 mA	maximum load current in sleep mode
Idisconnect	100 uA	maximum load current in disconnect mode
fAC_min	50 Hz	minimum frequency of AC noise test
fAC_max	25 kHz	maximum frequency of AC noise test
tdropout	0.1 s	time of the voltage drop-out test
tsleep	5 s	maximum delay before entering sleep state
tLED_sleep	5 s	period the status LED sleeps
tLED_delay	60 s	delay until the LED goes to sleep

Voltage and Over-voltage Protection

Source ports and bidirectional ports sourcing power shall supply power with the voltage in the range V_{min_s} to V_{max_s}, as measured on the source connector. The source shall supply at least V_{min_s} when sourcing its maximum rated power. Load ports shall operate normally when receiving power whose voltage is in the range of V_{min_l} to V_{max_l} as measured on the load connector or plug. AC adapter ports are permitted to source power with the voltage in the range V_{min_s} to V_{max_a} as measured at their plug.

Loads are expected to operate normally over a wider range than source ports normally supply to let them operate with 19.5 V-style AC power adapters and, at the low end, to accommodate an IR voltage drop between the source and the load. The normal source upper voltage limit is constrained to a lower voltage than V_{max_a} to prevent damage to legacy devices attached to an ODG bus.

All load ports and bidirectional ports receiving power are expected reduce their current draw and go into sleep mode when the received voltage drops below V_{min_l}. This standard makes a distinction between the communications circuits of a device and circuits related to its primary function. The communications circuits are expected to operate at a lower voltage than the primary circuits so they can report an under-voltage condition even when the primary function cannot be performed. If implemented on a device, communications circuits are expected to operate normally when the received voltage is in the range V_{min_c} to V_{max_l}. When receiving less than V_{min_c}, devices shall enter disconnect mode and draw negligible current.

Note that devices are expected to delay entering the sleep or disconnect states to prevent the interruption of normal operation during temporary voltage dropouts (see below). The delay before entering a sleep state must be at least t_dropout but no longer than t_sleep.

All ports except AC adapter ports shall implement sustained over-voltage protection to V_ovp. This protection applies to both negative and positive poles as well as the communications signal. During the over-voltage, the device is not required to maintain normal operation (Functional Status 1), but may enter a less functional state (Functional Status 3). When the voltage returns to normal limits the device must resume normal operation (Functional Status 1) with no intervention by the user. Details of the required conformance test are given in Appendix C. that include stress pulses of various voltage peaks, durations and repetitions.

Source ports and bidirectional ports operating as sources shall limit the voltage overshoot to less than V_ovs when a load is disconnected. Test conditions are given in Appendix C and require the disconnect of the maximum rated load for the port.

Load ports and bidirectional ports receiving power shall limit conducted emissions to voltages less than V_cond. This is normally only relevant to inductive loads. Test conditions are given in Appendix C.

Current

The maximum current supplied by a source or bidirectional port, or consumed by a load through a connector shall be I_max. A source or bidirectional port is not required to provide any minimum level of current.

A device in sleep mode shall reduce its current consumption to I_sleep or less. A device in disconnect mode shall reduce its current consumption to I_disconnect or less.

Devices requiring current in excess of I_max or sources providing more current may exceed these current limits if they provide alternate means of connection such as screw terminals. Such devices shall conform to all voltage requirements and shall describe their wiring requirements in their documentation. Sources shall provide over-current protection suitable to protect the wiring and connections as described in their documentation. They shall also implement the sleep mode and disconnect mode current limits.

All devices supplying power through a source or bidirectional port shall implement communications over those ports. Devices offering non-standard connections such as screw terminals shall include terminals for communications that is compatible with the connector-based communications signaling.

More than one power source can be attached to a bus. To allocate current fairly between sources, all sources shall implemented droop control. [Details TBD.]

[Should we permit non-droop control sources on a bus? A device could identify itself as either being droop controlled or not. A not droop control source would not permit any other sources on the bus.]

Communications signaling between sources is used to limit the the maximum current flow through the bus wiring. All source devices shall have the capability of limiting their maximum current output to a level designated in a configuration message from the current bus manager. By default, the maximum current from all sources shall be limited to I_max. The maximum current from all sources in an installation shall be configurable to level higher than I_max through appropriate management software if a trained installer can ensure that the wiring can support the higher current. It is the responsibility of the current bus manager (see TBD) to ensure that all power sources are appropriately configured to the designed bus current limit.

The maximum current of an AC adapter port is not defined by this standard. It is the responsibility of the load to ensure that it does not draw enough current to damage its own connector or wiring.

Over-current Protection

A port sourcing power shall include over-current protection to prevent damage to the device in the event of a fault that shorts the positive pole to the negative pole for an unlimited time period. The over-current protection from a source or bidirectional port shall automatically reset so the device resumes normal operation when the fault is resolved. An over-current condition shall be reported using communications as described in section TBD.

[ Must resume normal operations in TBD seconds. How is resume initiated? ]

The over-current condition shall be reported by an LED located near the connector as described in Port Status Indicator.

If a load device supports communications, it shall report the maximum normal operating current of the device at the minimum supply voltage and shall also report the limit and duration of any temporary current surges the device may experience such as those related to motor starts, using the protocols described in Communications.

A load port is recommended but not required to provide over-current protection from faults internal to the device that would result in current draws in excess of its rated maximum. Over-current protection for a load may be implemented with a fuse. A replaceable fuse is recommended but not required. If the device supports communications, it is recommended that the communications be implemented such that communications remain operating even when the primary over-current protection has disabled the device so the device may report the fault condition.

A fault that shorts the communications pin to either pole will disable communications but not have any permanent effect on the device such that communications will resume normal operation when the fault is resolved.

[ Should we permit temporary current overloads in excess of 8 A? ]

[ Motor start overloads ?? ]

[ Capacitor overloads ?? ]

Grid Overload and Fault Isolation

Many circumstances can cause the overall power demand from loads to exceed the power available from sources. In the simple case with a source supplying power to non-communicating loads wired to a common bus this can manifest as an over-current condition analogous to blowing a fuse on a normal AC grid. The source must use its normal over-current protection and disconnect current flow.

A managed grid like ODG has more options. If its sources have multiple ports that can controlled independently, it may be able via current monitoring to isolate the overload to one port and disconnect that port while leaving the others functioning. If the load on each port is below I_max but the aggregate exceeds the current capacity of the source, it may choose to shut down a subset of its ports on a priority basis.

Communicating loads give the source or sources more options. Among the parameters that a communicating load is required to supply is its normal power demand. The grid can selectively prioritize loads using administrative configuration.

[More TBD]

Reverse Polarity Protection

All ports shall implement reverse polarity protection so that the device is not damaged when connected to a voltage source of V_reverse for an indefinite period. When connected with reverse polarity, the device is not expected to function (Functional Status 3) but must resume normal operation (Functional Status 1) when correct polarity is re-applied with no intervention by the user required.

If a port supports communications, recommended that the communications logic be powered by a full-bridge rectifier so that it will continue to operate with the power poles reversed and can report the fault using the methods described in section TBD.

It is recommended [ Required?? ] that all devices implement an LED that will illuminate red with the power poles are reversed.

Drop-out Protection

Various conditions as when new loads are attached may cause the bus to temporarily drop to a lower than normal voltage, a condition known as voltage dropout. Load devices are expected continue to operate normally (Functional Status 1) during voltage dropouts. During the test to verify the device, voltage is lowered to V_dropout for a period t_dropout and then return to normal. Details of the conformance test are given in Appendix C.

Noise Immunity

Load devices are expected normally (Functional Status 1) when an AC voltage of V_AC is superimposed on their normal supply voltage. This AC voltage can be any frequency between f_{AC_min} and f_{AC_max}. Details of the conformance test are given in Appendix C.

Slow Rise and Fall of Supply Voltage

Load devices are expected to transition through levels of function when the voltage they receive increase or decrease slowly as might be expected from a battery charging or discharging. From 0 volts to V_{min_c}, the device is expected to be in the disconnect state drawing negligible current. It may have no apparent function. From V_{min_c} to V_{min_l} it should be in the sleep state such that communications is operating normally (Functional Status 1) but the devices primary function may not be available (Functional Status 3). It must enter normal operation (Functional Status 1) when the voltage enters the normal operating range of V_{min_l} to V_{max_l}. The reverse sequence must occur when the supply voltage begins to decrease. As part of its drop-out protection the device is may delay changing to a higher functional for a period no more than V_dropout. While the voltage is dropping, the device must enter the sleep or disconnect state within t_sleep. Details of the conformance test are given in Appendix C.

There is another test requiring that the device maintain the proper operational state when the supply voltage fluctuates to progressively lower level and then returns to normal in steps with a period of t_sleep or greater.
Again, test details are provided in the appendix.

Starting Profile

[TBD - is this test needed?]

Transient Immunity

Various conditions may trigger brief voltage spikes that exceed the normal over-voltage protection for brief periods. In the the automobile environment, the requirements for transient immunity are given by ISO 7637.

[ODG requirements TBD.]

Open Circuit

Disruptions during wiring and maintenance of a grid may cause some circuits to open in unexpected ways. For example, the communications signal may be connected when either the positive or negative power circuit is disconnected. The device shall be undamaged but potentially non-functional (Functional Status 3) under all combinations of attached wiring.

A load device that implements communications but does receive the normal communications recessive signal shall assume that it is connected to a non-standard power source. It should perform all its normal functions. One exception is a device that implements PAYG whose payment token is expired. In this case it should indicate this condition to users using the methods in section TBD.

Details of the conformance test are given in Appendix C.

Galvanic Isolation

When multiple devices are powered by a single 12V Link, they may observe different voltage levels on each pole relative to the source reference. Devices are typically implemented to use the negative pole as a signal ground reference and therefor differences between the negative poles on each device may result in differing signal ground references in each device. If these devices are connected together with another cable besides the 12V Link power cable such as a USB communications cable that connects the grounds, unexpected current may flow through the ground connections. This condition is commonly referred to as a ground loop. The ground loop will result in noise that disrupts the communications signals and in extreme cases may result in damage to the equipment.

[Need illustration]

To avoid this problem, when any two devices powered by a signal 12V Link are connected with another cable, at least one of them must use a galvanic isolator on its power cable. This is a device that electrically isolates the power poles across the device so that the down stream side is electrically floating relative to their common reference. This isolation breaks the ground loop and solves the problem.

[Need illustration]

Cold Start

Grids based on the 12V Link may experience a temporary overload when power is first applied to the 12V bus to charge input capacitors and initiate other start-up functions such as refrigeration motor starts. Devices without communications but with the ability to control their input current through a FET switch are recommended to delay their power-on with a randomly selected delay up to TBD seconds to minimize the chance of a startup overload.

Loads with communications and the ability to control their input current shall attempt to establish communications with the current bus manager before admitting start-up currents. If communications can be established with the bus manager, they shall request power from the bus manager and delay their start until a power grant message has been received from the bus manager. The bus manager shall stage power initiation to reduce the chance of an overload. More details of the protocol are contained in the Communications section.

Port Labeling

[ Use ODG label with separate icon for load, source or bidirectional. Power consumption or supply in Watts. TBD]

Port Status Indicator

All devices implementing a 12V Link port shall indicate the status of that port with a multicolor LED using the following predetermined patterns. Load devices only need a two-color LED that can display green, red or amber (both green and red on). Source-only devices need a blue / red LED. Bidirectional ports need an LED that includes red, blue and green.

To minimize power consumption, the LED shall be in an awake state or a sleep state, states that are independent of the overall device state except that the LED shall be off and consuming no power when the device is in the disconnect state. In the awake state the LED is illuminated but indicating additional status details via a pulsing pattern to repeats in a period of t_{LED_sleep}. In the sleep state the LED is off but awakens to display its status in a period t_{LED_sleep}, the inverse pattern of the awake state. The LED shall be in its awake state whenever the device transitions into its normal operating state. The LED shall transition to its sleep state after t_{LED_delay}, even if the device itself is operating normally.

Color	Blink Dots	Description
Green	1	load receiving normal power
Green	2	load receiving normal power but without communications
Amber	1	load or source voltage below Vmin_l (sleep)
Amber	2	load with insufficient priority for power
Amber	3	load or source PAYG disabled
Red	1	source over-current
Red	2	reverse polarity
Red	3	overvoltage or device malfunction
blue	1	source suppling normal power
blue	2	source suppling normal power but without communications

Communications

Open DC Grid is a managed grid and some functions related to management require communications. The different link types have different forms of communications that best fit the intended applications of that link so the ODG approach is to support different forms of physical and low layer protocols with semantically consistent upper layers so that the different protocol stacks and be easily bridged when necessary between different link types.

Communications for the 12V Link is based on the LIN Bus at the physical layer, a unique link layer that is optimized for the target applications, a subset of CoAP in the middle layers and an application layer homologous to a subset of IEEE 2030.5 (SEP 2) but again, optimized for ODG. Its key features include:

• Implementation on very small microcontrollers
• Negligible power consumption for an inactive bus or inactive nodes
• Automatic address assignment and configuration
• Peer to peer communications
• Automatic data rate adaptation of 1 - 20 kbps
• Operation to 40 m without twisted pairs

The following description of the ODG 12V Link communications approach is based on the traditional 7-layer OSI communications model. Some of the layers are different from existing standards so the entire stack here is referred to as ODGTalk, just to give it a unique label.

Physical Layer

The physical layer is based on the LIN bus. Implementations can use existing LIN bus transceivers, though the selected transceiver must support the full voltage range of V_{min_c} to V_{max_l} for normal operation and V_reverse to V_ovp over-voltage and reverse-voltage protection. For the following, the voltage observed at a device as the difference between the positive and negative poles is referred to as V_bus. The two LIN signal thresholds are:

• 80% to 100% of V_bus recessive signal (serial high)
• 0% to 20% of V_bus dominant (serial low)

One node, designated the bus manager, pulls the bus high through a 1K ohm resistor and FET switch to make the default state be the recessive state. Transceivers pull the bus down to the dominant signal for signaling.

When used with the 12V Link, all sources shall be capable of assuming the bus manager role and activating the associated pull-up. When multiple sources are connected to a bus, a link layer arbitration protocol selects which of the bus manager candidates assumes the role. Another link layer protocol permits fail safe operation to shift the bus manager role if the original bus manager disconnects for whatever reason.

Signal timing is based on the current data rate for the bus which can vary between 1 kbps and 20 kbps. The bus manager selects the bus rate. Other nodes use a synchronization mechanism to automatically match the current received bus rate when transmitting. It is possible that a node can fail to synchronize due to the physical limitations of a particular installation. In that case, it can request a slower data rate by forcing an extended dominant (break) condition on the bus. The current bus manager must observe this request and initiate transmission at a slower data rate. This process repeats at slower and slower data rates until all nodes synchronize. If synchronization cannot be achieved at 1 kbps, communications is declared non-operational and nodes revert to their non-communications mode.

Link Layer

The ODGTalk link layer includes protocols to assign select the bus manager, select the data rate, assign local addresses and transmit transmit data frames. The data frame format has been chosen to take advantage of power-saving features built into many micro-controllers to minimize the power consumption of individual nodes and for the ODGTalk network as a whole.

Whenever a node joins the network by being plugged into a 12V Link bus or by receiving power from another source it performs the following steps in sequence.

1. Master Arbitration

When a node capable of being a master joins the network, it must decide if there is an existing master, in which case it should join as a normal node, or if there is no master, it should assume the master role. Upon initialization such a node first observes the bus. It observes network traffic, there must already be a master so it joins as a normal node. If there is no traffic observed, it waits for a random delay of TBD and transmits a wakeup probe, which is a break of TBD seconds. If a master is already active, the master will respond to the wakeup probe and the candidate joins as a normal node. If there is no response after TBD seconds, it retries the wakeup probe. If there is not response after TBD retries, the candidate activates its master pull-up and assumes the master role.

[ Note that transceivers have a weak built-in pull-up so observing a recessive state is not sufficient to ensure that a master already exists. ]

2. Data Rate Selection

ODGTalk permits dynamic adjustment of the data rate to deal with changing physical conditions on the bus. Each frame transmitted on the bus includes a synchronization field similar to LIN that enables the receiver to automatically set its receive data rate using the timing in the synchronization field. When a device initializes, it observes the bus for activity and if found, it sets it data rate using the observed traffic. If there is no traffic, it sends a wakeup probe, a break sequence, like that used in the master arbitration. This will trigger a wakeup response from the master and the new node can then set its date rate.

Changing physical conditions on the bus can make data transmission unreliable and require the bus to slow down. Any node observing excessive data errors can request the bus to slow down. There are two methods to do this. If the bus is marginal, any node can send a data errors alert using normal transmission methods to the master. If this does not result in a slow-down sequence, a node can initiate a special slow down alert which is an extended break of TBD seconds. The master observes this event and initiates a slow down sequence. A slow down sequence is a broadcast message send to all nodes telling them to reinitialize their data rate.

[Note that the master hardware must be to determine the length of a break using counters or other mechanisms. - Details TBD.]

3. Address Registration

Each node on the bus has a logical bus address that is assigned dynamically. Once assigned, it responds only to messages to that address or broadcast transmissions which are sent to all nodes.
If the device hardware supports a hardware address filter, it can set its assigned address in that filter so that it only wakes on messages to that address.

Part the address registration sequence includes a flag that indicates if the device is using a hardware filter. If it is, the device will not receive a broadcast message. In that case, the master sends each such node a broadcast prime message instructing the node to temporarily disable the address filter until after the broadcast. After it receives the broadcast, it can resume using the filter.

Address registration occurs during node initialization, after the data rate has been established. Address 0 is reserved for the master. Slave nodes can use any of the 15 remaining address. A slave begins by choosing a random address between 1 and 15. To register an address, the registering node sends a registration request to the master using the address it selected. The master sends a registration response to that address indicating whether the registration was successful.
If another node has already registered that address, both nodes will receive the response. The node that previously registered that address can ignore the response. The failure response includes an alternative address that the device can adopt. It can also indicate that no more addresses are available in the unlikely case that 15 slaves have already registered.

The address registration message include the 64-bit IP6 address of the node. This can be queried by a proxy node to establish the IP6 address of the target node.

4. Data Transmission

Normal data transmission in ODGTalk uses a carrier sense multiple access with collision avoidance (CMSA/CA) mechanism. In this case, a "carrier" is just activity on the bus. A node can send a message to any other node at any time. To send a message, it first observes the bus by enabling data reception. If there is no activity on the bus, the node can send its message immediately. If there is activity, it just delays a random time of TBD seconds and checks again. It is still possible that two nodes can start transmitting at the same time causing a collision. All messages must assume the possibility of transmission errors and include a retry mechanism. Collisions are resolved using normal retransmission.

[TBD: receive your own message and watch for data corruption indicating a collision. Abort and retry.]

[TBD: wait for end of transmission by a timeout. Then delay a slot time based on the address.]

Frame Format

At the bit level, ODGTalk uses a normal UART format:

• 9-bits / character (9th bit ignored except for address flag)
• 1 stop bit
• no parity

The basic frame structure is similar to LIN:

• break (wake up all transceivers)
• sync (0x55 to establish the data rate through auto baud rate recognition if not already established)
• destination address and frame type (includes address flag)
• source address
• frame length
• message content (0 - 127 bytes)
• CRC (2-byte CCITT-16)

This frame structure is chosen to enable hardware optimizations built in to many micro-controllers. Some devices include auto baud rate detection registers. Most devices include address detection using the 9th bit called multiprocessor communications mode. Some devices also include hardware support for the calculation of the CRC.

With no bus activity, all transceivers are in sleep mode and all micro-controllers can be in sleep mode. Once transmission starts, all transceivers wake up. Micro-controllers with address-recognition hardware can stay asleep except for their address compare logic.

Network Layer

In normal operation, there is no network layer in ODGTalk. Nodes are incapable of directly communicating with nodes outside their own bus, and similarly, there is no direct way for an external network to directly access an ODGTalk node. This is for both simplicity and security. The physical span of an ODGTalk network is limited to the physical span of one 12V Link.

However, any node that has access to another network such as the Internet, can act as a proxy for nodes in ODGTalk. As part of joining the ODGTalk network, a node is required to register its IP6 address with the the bus manager. This creates a mapping between an IP6 address and the local address in an ODGTalk network. Any node on the ODTTalk network can query this mapping and thus proxy messages between the networks.

In addition, the link layer supports multiple frame types. One of the supported frame types can use 6loWPAN-style header compression and packet reassembly to create full IP6 frames. This capability is optional for 12V Link nodes.

Transport Layer

The core transport layer is based on a subset of CoAP (rfc7252) with exponential back-off. Confirmable and non-confirmable messages are supported.

[Details TBD.]

Session

ODGTalk does not have a session layer. All standard functions are idempotent so message de-duplication is not required.

Presentation and Application

[ This is still being discussed. Alternatives under consideration include a predefined register set with vendor extensions similar to Modbus that only supports basic get/set and a richer format based on ThingSet (opens new window). ThingSet supports retrieving a collection of data in both text and binary (CBOR) formats with selective retrieval and modification via FETCH and iPATCH. ]

1-Wire Communications

[ Laptop power adapters typically include an identification ROM in the power supply that can be read by the load using the 1-Wire protocol. Currently the content of these ROMs are all proprietary. We could propose a standard for this content. I think a smart load could attempt 1-Wire to determine if the source is compatible and if that fails, attempt to initiate ODG communications. ]