RS485 Modbus Communication with modpoll on Linux Returns Only Zero Values

I'm attempting to communicate with an RS485 modbus enabled device, using modpoll on a Linux based OS.

I built the RS232 to RS485 converter, from this site, and believe it to be working; you'll see my comments against the project too http://www.eeweb.com/project/circuit_projects...-rs485-converter-with-automatic-rx-tx-control.

My question is really a call for help, as I don't really know what's wrong I can't ask a direct question to fix it. At the moment, I can only ever retrieve '0' from modpoll, and I'm wondering if any one has some hints for me to get more.

I appreciate the question is a little light on details, so please feel free to contact me if needed.

Regards,
Jon

If you use a short cable the 120ohm resistor is a overkill on the two sides.
For long distance you need one at the beginning and at the end.
In my work they are never installed on the pcb bud only on the empty begin and end.
p.s. for testing by use of a 2 m cable i do not even use them.

Hi, Peter. Thanks for taking the time to reply.

I'd not used terminating resistors, but did try a 2M cable. Unfortunately it gives the same results, and by that I mean the output of modpoll is 0.

e.g.

# ./modpoll -m rtu -b 9600 -d 8 -p none -s 1 -a 1 /dev/ttyS0 -c 100 -1
modpoll 3.1 - FieldTalk(tm) Modbus(R) Master Simulator
Copyright (c) 2002-2011 proconX Pty Ltd
Visit http://www.modbusdriver.com for Modbus libraries and tools.

Protocol configuration: Modbus RTU
Slave configuration...: address = 1, start reference = 1, count = 100
Communication.........: /dev/ttyS0, 9600, 8, 1, none, t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, output (holding) register table

-- Polling slave...
[1]: 0
[2]: 0
[3]: 0
[4]: 0
[5]: 0
[6]: 0
[7]: 0
[8]: 0
[9]: 0
...

Is it possible for the device I'm polling to not be compatible with modpoll? n.b. Lot's of trial and error at the moment, so any suggestions are welcome.

Hi,
First on the interface that you use the termination resitor is fixed installed so make sure that it is not there.
second make sure that your slave is using address 1 keep in mind that 32 can talk at the same time, so listening at the wrong address is what i think is your problem.

Hi Jon

Are you sure the device is not expecting Modbus ASCII instead of RTU?

Do you have 2 of the RS232-RS485 converters? If so you could connect them between two COM ports/2 PCs to verify they are working correctly.

Also possibly connect modpoll between your rmaster PC and another COM port/PC (via RS232) to check if it is sending what you expect.

You could also use a demo version of a SCADA program, like Citect with Modbus to check if the problem is related to modpoll. (Some years ago you could run this for 15 minutes without a license.)

Good luck.

Hi,

I removed the terminating resistor, but it made no difference.

Is there a way to confirm the slave address? I'd assumed it was 1 as it's the only device. Can only one device have another address?

Hi Pieter,

I am sure the device is expecting RTU.

I've only one at the moment, so that test is not an option at the moment.

I'm looking in to SCADA options, thanks.