Asynchronous Multi-Node Bus (AMNB)¶
lbuild module: modm:communication:amnb
The AMNB (**A**synchronous **M**ulti-**N**ode **B**us) is a multi-master bus system, using p-persistent CSMA/CD to send messages.
One bus can be populated with up to 256 (logical) nodes. The nodes can be queried for data and they will respond like an SAB Slave, and can query data from other nodes like an SAB Master, or they can just broadcast a message. Each node can listen to all the responses and broadcasts and store that information for its purpose.
Action callbacks to query requests can be defined as well as universal callbacks to any transmitted messaged (Listener callbacks). As an optional advanced feature, error handling callbacks can also be defined, which fire if messages have not been able to be sent, or requests timed out or misbehaved in other manners, or other nodes query unavailable information.
Protocol Structure¶
Message without data:
SYNC: Synchronization sequence.CRC8: checksum for the headerADDR: Node address.TYPE: Message type .COMMAND: Command code.
1 2 3 4 5 6
+------+------+------+---------+------+
| SYNC | CRC8 | ADDR | COMMAND | TYPE |
+------+------+------+---------+------+
Message with <=28B data:
CRC8: checksum for the header and data.TYPE/LENGTH: Message type and length.
1 2 3 4 5 6 7 ...
+------+------+------+---------+-------------+--------------------+
| SYNC | CRC8 | ADDR | COMMAND | TYPE/LENGTH | ... <=28B DATA ... |
+------+------+------+---------+-------------+--------------------+
Message with <8kB data:
CRC16: checksum for the data.DATA: Up to 8kB payload (nodes may support much less though!)
1 2 3 4 5 6 7 8 9 10 11 ...
+------+------+------+---------+------+--------+-------+-------------------+
| SYNC | CRC8 | ADDR | COMMAND | TYPE | LENGTH | CRC16 | ... <8kB DATA ... |
+------+------+------+---------+------+--------+-------+-------------------+
Message Types¶
| Type | Meaning |
|---|---|
| 000 | Data broadcast by a node |
| 010 | Data request by a node |
| 100 | Negative response from the node (NACK) |
| 101 | Positive response from the node (ACK) |
When transmitting, the second bit determines, whether or not to expect an answer from the addressed node. To just send information without any need for acknowledgment, use a broadcast.
When a node is responding, the second bit has the following meaning:
true: Message is an positive response and may contain a payloadfalse: Message signals an error condition and carries only one byte of payload. This byte is an error code.
Electrical characteristics¶
Between different boards CAN transceivers are used. Compared to RS485 the CAN transceivers have the advantage to work without a separate direction input. You can just connected the transceiver directly to the UART of your microcontroller. These are identical to the SAB CAN electrical characteristics. You have to use the CAN transceivers, otherwise it cannot be determined, if the bus is busy or available for transmission.
Options¶
with_heap¶
Allow large messages to allocate on the heap.
Default: True
Inputs: [True, False]