Created Date: August 09,2007

Created by: bcarlton

I have used MODBUS quite a bit with the AD line. I have never played with the 'timeout ' function.

When the communication has started, various responses are expected withing a variable but limited time period. If there was no timeout (unlimited time) the process (either initiating a communication or responding to a request) would wait forever if something got messed up in the characters being exchanged.

Using the timeout, when the selected time goes by without an appropriate response the communication system resets (it doesn't reset the PLC), giving an appropriate error and becomes ready either to respond to another query or to handle the next MWX or MWX command.

So, if you are using the normal MODBUS features just leave it at the default. it would be extended if the communication were possibly going by a number of intermediate hops (a rack on the far side of the country connected by modem).

{Edit] Yes the 'busy ' bit will be set until the reset timeout has taken place. I 'v had this if one of the units becomes disconnected.

Created Date: August 09,2007

Created by: clintonp

Thanks so much for the prompt reply!

I'm not sure that what you 're referring to is the Reset Time-out. And, to my knowledge, it doesn't have a default value.

The implication to me of the description of the Reset Time-Out ( "...after the port goes from the BUSY state to the IDLE state. ") is that if the port doesn't become busy often enough, the DCM gets reset. The only thing that I can imagine causing it to be in a BUSY state is that it's been polled by the Master. So if the Master only polls, for example, every two minutes, this will always trip (longest wait is 99 seconds). I have no idea what the user's polling rate will be, but it seems that if it's more than 99 seconds, I should turn it off. If they poll more frequently than every 99 seconds, then it would be OK to turn it on.

Make sense?

Paul in WI

Created Date: August 09,2007

Created by: clintonp

I'm creating screens that will allow the user to set the parameters of a D0-DCM, so that they can (theoretically) connect to whatever MODBUS RTU network they might have in place.

I pretty much understand all the parameters except Word +24: Port 1 and 2 Reset Time-out. The description in the DCM manual is, "The communication port will reset after the specified time after the port goes from the BUSY state to the IDLE state. " I'm not clear about what this means, how this works, and how it serves me to use it.

This DCM will be a MODBUS slave. Are they saying that if this 'station ' is not polled within the specified time, the DCM will reset?

What exactly happens when the DCM is reset? And, when it IS reset, is it unavailable for a time? Does Port Busy go active until the reset is complete?

I might fill a thimble with what I know about MODBUS RTU. How often should I expect a poll?

Thanks in advance for any insights,

Paul in WI