ModBus / RS485 timing / RedLion G310

Hi All,

I have a RedLion G310 panel which talks to a number of devices via Modbus. The G310 has both RS485 ports installed because one Modbus loop uses a single stop bit and the other two stop bits.

I have a remote PLC that is interrogated on the 'single' stop bit loop. This loop has most of the other devices. The remote PLC is connected via a radio link with an intermediate repeater. Therefore it takes some time for the ModBus packet to reach the remote PLC and sometime for the answer to be returned.

I am apparently getting 'time out' errors when the G310 does not get a returned Modbus frame in time. Or possibly the remote PLC packet is colliding with other traffic because the radios might not be monitoring the loop traffic. What happens is that the G310 sets all the variables requested in the Modbus packet to zero until a valid packet is returned.

I don't need the remote data very frequently and I have set the G310 to poll as slowly as I can.

Question; Is there anyway to tell the G310 NOT to set all variable to zero but to use the old values if a Modbus packet does not arrive in time?

Unfortunately I can't put the remote PLC on the loop with less traffic because it does nto work with two stop bits and there is no other slots available for more RS485 cards.

Ideas would be welcomed. Thanks

1) The 'timeout' setting is the one that determines how long the master waits for a reply until it gives up and goes onto the next slave poll. I'm sure the Red Lion has a timeout setting.

2) I don't know whether there is a setting to use 'stale' or 'last' data in the variable buffers, rather than zero, if a timeout occurs, but I know that there are lots of people who wish there masters did that because they'll get what appears to be valid data but it's old stale data due to a comm link problem. They've had to resort to alogrithms like comparing successive floating point values to see if they're identical, because the probability of getting a pressure 17.823946 bar (out to the 6th decimal place) 3 times in a row is pretty low.

Maybe a Redlion guy can answer that.

Thanks for your reply - there is a Comms Delay setting that can be set up to 60,000ms or 1 minute. But that sets the time between 'Polls' it does not seem to set the time that a reply is expected after the device has been polled. I already have that on maximum setting.

AdrianEL said:

Thanks for your reply - there is a Comms Delay setting that can be set up to 60,000ms or 1 minute. But that sets the time between 'Polls' it does not seem to set the time that a reply is expected after the device has been polled. I already have that on maximum setting.

Click to expand...

When you choose the Modbus Universal Master, set the Slave Timeout under Driver Settings. This will apply to all the devices connected to the RS-485 Port. I think this will give you the flexibility you need.

Thank you very much - that looks promising.