I have been trying to communicate to an Allen Bradley PLC a Micrologix 1100, using the RS-485 port configured as Modbus RTU Slave and getting this read by a Lantronix xPress DR+ so i can get the data to an HMI. The thing is that there is not much information on how to get the PLC fully configured for Modbus RTU Slave, or how to connect it to a RS-485 network (physically). I got the NC01 RS-485 cable connector from Allen Bradley and i connect it to the Lantronix as a two wire RS-485 connection, but got nothing, i even tried putting termination resistors (122 ohms) on both ends, but i don't know if i am doing it right, since Allen Bradley does not use TX+ or TX- it uses A or B, so anyway i tried every combination with the A and B and nothing.

In the software i configured in channel configuration channel 0 for Modbus RTU Slave and double checked the communication settings to match the Lantronix in speed and all that stuff, i also configured the data table, but nothing. I am verifying with ModSCAN32 and i see the Lantronix lighting, polling the PLC but i get no response from it. Please help!

 

>I have been trying to communicate to an
>Allen Bradley PLC a Micrologix 1100,
>using the RS-485 port configured as
>Modbus RTU Slave and getting this read
>by a Lantronix xPress DR+ ...

Did you connect the ground signal as well? Since the DR+ has a floating ground, and I think so does the MicroLogix, then you will need to connect the RS-485 signal grounds.

You may also need external bias resisters. I haven't used the DR+ so don't know if they include those. These keep the +/- lines from floating & generating lots of garbage when nothing is transmitting.