Protocole used to exchange data between server and clients

The communication between server and clients uses RS232 frames.

4800 Bauds
8 data bits
No parity
1 stop bit

These frames are sent either by the server or a client.

The RS232 frame

STX

ADD

SLN

CMD

PAY1 .. PAYn

CHK

Any other bytes are ignored until a new STX

- STX

This field is one byte long with the STX character (0x02)

- ADD

This field is one byte long and is divided in two nibbles:

MSB nibble is the sender address (4 bits) value: 0 to 15
LSB nibble is the destination address (4 bits) value: 0 to 15

0 : Server
1 : 433MHz Receiver
3 : Relays I/O card
4 : PLC master card

- SLN

This field is one byte long and is divided in two nibbles:

MSB nibble is the Frame sequence number (4 bits) value: 0 to 15
LSB nibble is the length of the payload (4 bits) value: 0 to 15

The sequence number is incremented after sending a new frame. The sequence number counts from 1 to 15.
The sequence number 0 is only used once after start (for the first frame) and indicates a reset of the sender.

The sequence number is specific to the server and each client

The length of the payload may be 0 (no payload) to 15 bytes.

- CMD

This field is one byte long. It gives the kind of command send to the destination address.

- PAY

This field is from 0 to 15 bytes long. It holds the data corresponding to the command.
The data bytes are binary: each byte may hold any value from 0x00 to 0xff

The length of this field must match the length given in the SLN field.

- CHK

This field is one byte long. It holds a checksum value of the frame.

The checksum is computed as follows:
Summing all bytes of the frame (excluding STX) will give a value of 0

ADD + SLN + CMD + PAY1 + ... + PAYn + CHK = 0

Connection of the clients to the server

All clients are chained in the same RS232 line (daisy chain):

 ----------------------------------------------
|                  Addr=0                      |
| RS232TX          Server              RS232RX |
 ----------------------------------------------
     v                                   ^
     |    ----->-----     ----->-----    |
     |   |           |   |           |   |
  -----------     -----------     -----------
 |  RX   TX  |   |  RX   TX  |   |  RX   TX  |
 |   Addr=1  |   |   Addr=2  |   |   Addr=3  |
  -----------     -----------     -----------

When a client receives a frame in its RX line, it has to analyse the ADD field:

If the ToAddress is its address, then the frame is processed and answered to TX line(mandatary). The ADD field is swapped so From and To addresses are exchanged. The other fields are changed depending of the process. The checksum is re-computed.
If the ToAddress is not its address, the frame is forwarded to the TX line without any change. The checksum is not changed.

Mandatory commands

All clients must answer to a ping request and to an ident request. These frames are used to know and identify the list of available clients in the daisy chain.

- Ping

The ping is CMD=1

Exemple of a ping frame sent by server (address=0, sequence=4) to 433MHz receiver (address=1):

02 01 41 01 C6 F7

Answer to the ping frame (sequence=15):

02 10 F1 01 C6 38

The payload includes 1 byte. The value of this byte is chosen by the sender and must be copied in the answer without any change

The checksum of the answer frame must be re-computed.

Note: The server sends a ping frame to itself to check the continuity of the RS232 loop. This ping frame will be relayed by each client up to the server.

- Ident

The ident is CMD=2

Exemple of an ident frame sent by server (address=0, sequence=5) to 433MHz receiver (address=1):

02 01 50 02 AD

Answer to the ident frame (sequence=1) with the string "RX V2":

02 10 15 02 52 58 20 56 32 87

There is no payload in the ident request.

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