ml 1100 as a modbus rtu slave

Hi all,

Wondering if anyone has successfully setup an ml 1100 as a modbus rtu slave?
I tried today with no luck. I'm certain I've got the CH0 settings properly (modbus slave, 19.2k baud, no parity, 1 stop bit, no handshaking 485), and I have verified these settings on the master as well. The one area I am not certain on is the communication cable, the 1763-NC01 cable - I have no manual for this, and couldn't find one on the net. I am only using 2 terminals on it (A and B - I will eventually ground it as well, but right now I just want to see some communications). I assume the other terminals on this cable are for other purposes not related to a 2 wire rs485 system?? Distance is not an issue - the devices are within 1 foot of each other.

Any suggestions?

What is the Master device ?

Because you can't get any two vendors to agree on the meaning of "A" and "B" in EIA-485, the standard method everyone uses is "if it doesn't work, flip the wires".

"Two wire" EIA-485 is not really just two wires; there absolutely has to be a common reference, and that's the COM wire on the 1763-NC01 cable.

Many systems use Ground or a cable shield as their common reference, so if your Master device doesn't provide an isolated common, just ground (or attach to shield) the COM pin on the 1763-NC01 cable.

I have successfully used the MicroLogix 1100 as a Modbus slave many times on both RS232 and RS485 wiring systems.

As Ken stated, it should be reasonably painless to set the ML1100 as a Modbus Slave. We've used them as Modbus master several times without issue.

I've attached the pin out for the NC01 cable if it helps...

Rob

Thanks for the feedback guys. The master device is an "addme3 jr" embedded server; here's a link--> http://csimn.com/CSI_pages/AddMeJr.html

Ken- I tried flipping the A/B wires (with no luck), but I did not ground the common (the master has just 2 terminals, no isolated common). I'll try that approach next. Thanks again!

Just to let anyone interested to know, the problem has been resolved. It turned out there was a problem with the rs 485 port on the master controller. Once I swapped it out (and grounded the COM pin as Ken suggested) everything is fine.
Thanks for the help folks!

-Mike

Ken Roach said:

What is the Master device ?

Because you can't get any two vendors to agree on the meaning of "A" and "B" in EIA-485, the standard method everyone uses is "if it doesn't work, flip the wires".

"Two wire" EIA-485 is not really just two wires; there absolutely has to be a common reference, and that's the COM wire on the 1763-NC01 cable.

Many systems use Ground or a cable shield as their common reference, so if your Master device doesn't provide an isolated common, just ground (or attach to shield) the COM pin on the 1763-NC01 cable.

I have successfully used the MicroLogix 1100 as a Modbus slave many times on both RS232 and RS485 wiring systems.

Click to expand...

Would you share your rs232 setup for modbus ?