I'm looking into implementing Modbus function
code 8, sub function Code 00.

The Modbus Application Protocol Spec V1.1a shows the Data section of the message to be N x 2 bytes in size, yet the message does not contain any additional byte count information. This may be ok for Serial ASCII which wraps each message in special characters (':"<message>"\n") but Serial RTU does not. It is unclear to me how a
Modbus Slave could determine the end of such a message.

Does anyone have any input on this? could this be a typo in the spec? All additional function code 8 sub function codes show fixed size (two
byte) Data sections.

Thanks.

 

FYI: Many devices do not support this function in either master or slave mode.

Each subfunction has it's own special definition for the packet.

From the book:
"Most of the diagnostic queries use a *two–byte* data field to send diagnostic data or control information to the slave."

Function 8, sub function 0000 only has a fixed length, 2 bytes long, data field. Therefore there is no need to send 'N' and it should be easy to determine the end of the packet.

FYI: One of the methods used to determine the end of the packet is to use the packet timeout which is typically 3.5t (byte-time) (and a fixed time for fast baud rates).

-Max

 

Hello,

In Modbus RTU the interval of at least 3.5 character times marks the end of the message. A new message can begin after this interval.

Regards,
Andrzej
www.modbus.pl

 

A more complete discussion here:

http://groups.google.com/group/NModbus-discuss/browse_thread/thread/d642162d5f68a36c