Automation Direct DL06 modbus slave

[JustinW]

Hi,

I'm trying to setup an Automation Direct DirectLogic 06 (D0-06DD1) as a modbus slave, taking commands from a PC. The PLC is connected to the PC over port 1 with the standard programming cable, and I'm able to successfully read the digital inputs; however, I cannot update the digital outputs. I've tried both in TERM and RUN mode with no luck. I wrote a simple ladder program to change an output based on one of the inputs, and again I can successfully read the value of the output as it changes. Is there anything in particular I need to do so that digital outs can be updated via modbus RTU commands? Or maybe I'm missing something else entirely?

Thanks,
Justin

example program:
STRN X0
OUT Y1
END

 

[bernie_carlton]

Similarly to controlling outputs from a display I write to 'C' bits then in the program transfer that status to an output.

 

[icky812]

All communications are done at the beginning of the scan cycle. This means that if you are using the output coil in the program any change by the communications will get "over-written" by the status of the rung that the coil is on. "The last status of an output always wins."

As Bernie said, use "C" bits in parallel with whatever status bits you use to fire the output rung containing the coil in your PLC program.

 

[JustinW]

Bernie and icky812,

Thanks so much for the suggestions! When I initially tried your suggestion of using the internal control register, I still couldn't get it to work. This didn't make any sense, which led me to evaluate my Modbus commands more -- which turned out to be the root of the problem. I'm using a python library called modbus_tk (http://code.google.com/p/modbus-tk/), and my write commands were ill-formed. Fixing that, I was able to control the digital output lines on the PLC using the control relays or just directly (assuming the PLC program wasn't over-writing it, as mentioned by icky812). Thanks so much for your help! Now to figure out how to use the counters on this thing.

Justin