Brief Outline of the Comms Protocol (”STMbus”)

The STM requires a communication protocol to exchange commands and data with the operator interface on the host PC, and will be known hereafter as ‘STMbus’.

 

Request and Response

 

Commands and data are encoded into formatted message frames and sent via TCP/IP over a dedicated ethernet link. Communication between the STM and host PC consists of pairs of messages, a request message and a response message. Either party may initiate a request, but every request must have an acknowledgement response from the other.

 

Message Frame Format

 

I have decided to base the Message Frame on ASCII-mode MODBUS (with a few differences, detailed below).

 

Frame Start	Function Code	Data Length	Data	LRC	Frame End
1 char, ‘:’	2 chars	4 chars	0-65535 chars	4 chars	2 chars, CR+LF

 

Encoding

 

Each byte in the message frame is encoded as 2 chars e.g. 0x4D becomes encoded as 0x34 0x44 (‘4’ ‘D’). This encoding, with the printable ‘:’ denoting the start of a frame and a CR/LF denoting the end, was chosen to make debugging and development easier by allowing the use of text-manipulation tools. The loss of efficiency is not regarded as significant due to the speed of ethernet and the relatively small volumes of data being exchanged.

 

Field Description

• Frame Start indicates the start of a message frame and is a single colon (:).
• Function Code indicates the action or service required. A response message sets this to the same Function Code in the original request message. If the response needs to indicate an error condition, the MSB of the Function Code will be set to 1.
• Data Length is the number of encoded bytes (chars / 2) in the Data field.
• Data contains the parameters, arguments and other information associated with the Function Code action.
• LRC (Longitudinal Redundancy Check) is an error-checking field. It is the result of a computation performed on the bytes of the message frame before they are encoded, excluding the Frame Start and Frame End. It is calculated by successively adding the bytes together, discarding any carries, and then two’s complementing the result.
• Frame End indicates the end of the message frame, and consists of a carriage return (0x0D) followed by a line feed (0x0A).

Differences from MODBUS ASCII-mode

• The Data Length field has been extended to 4 chars (2 bytes) and is always present in the message frame (instead of being an optional part of the Data field).
• The maximum length of the Data field has been extended to 64k chars (32k bytes).
• The LRC has been extended to 4 chars (2 bytes)