On page 14 of the "MODBUS over serial line specification and implementation guide V1.02", there is the "Control and Waiting" state.

If the bus participant receives a character in this state, he has to flag the frame as NOK. But shouldn't he also restart the t3.5 timer? Otherwise the preceding silent interval is not guaranteed.

Am I missing something?

Regards
Timur Celik

 

I think you're right. You've to restart t3.5 each time a character is received during "control and waiting".

This may result in a dead lock if a faulty device is connected to the bus that spams around all the time. A proper Modbus client should notice that.

p. 14:
"<i>In RTU mode, the communication link is declared in "idle" state when there is no transmission activity after a time interval equal to at least 3.5 characters.</i>"

suggestion for figure 14:
<pre>Character received
/ flag = frame NOK and restart t3.5</pre>
I'm curious to read what the experts say