I am getting a timeout message on both an IDEC and productivity1000 plc when trying to read from a dfrobot rs485 temperature and humidity sensor. I can see the response but the PLC cant seem to recognise the response. Has any one got some suggestions for me to try? Yellow is A+ to 0vdc, blue is B- to 0vdc, purple is A minus B.

 

I am getting a timeout message on both an IDEC and productivity1000 plc when trying to read from a dfrobot rs485 temperature and humidity sensor. I can see the response but the PLC cant seem to recognise the response. Has any one got some suggestions for me to try? Yellow is A+ to 0vdc, blue is B- to 0vdc, purple is A minus B.
View attachment 2255

I am getting a timeout message on both an IDEC and productivity1000 plc when trying to read from a dfrobot rs485 temperature and humidity sensor. I can see the response but the PLC cant seem to recognise the response. Has any one got some suggestions for me to try? Yellow is A+ to 0vdc, blue is B- to 0vdc, purple is A minus B.
View attachment 2255

Hi i am trying to improve my understanding of industrial networks and troubleshooting. Lets cover the basics:
1 Is it modbus ascii or rtu?
2 is the reply (second wave) well within the timeout range?
3 would it be helpful to write digital values on the diagram? -is scale good enough for that? Deciphering analogue display to digital values i believe is the key
4 is diagram taken near the sensor or plc?
5 is it possible at all to capture logical values at plc or is the reply just not valid/ time out?

Regards

 

1: rtu https://www.dfrobot.com/product-2279.html
2 yes timeout is 500ms, on the oscope it shows about 5ms
3 I don't have digital decode option, but when I put the hot air on the sensor the response looks like it changes
4,5 it is a bench test, 1m cable, signal measured at the plc

Reversing the a b wires or changing parity stops the response, so parity and polarity are correct. Stop bits doesnt affect the response. Requesting 10 words instead of 2 in a longer response as expected.

 

Since you're correctly measuring your A+ and B- signals with respect to 0V common-mode reference using the oscilloscope, I assume you understand how RS-485 signaling works. So just to confirm, have you connected the 0V reference (a.k.a. com, gnd, ref, sg) of the IDEC or productivity1000 plc to the sensor's GND wire?

If you have, then I recommend trying a computer with Modbus simulator software and a USB to RS-485 adapter. Here are some Modbus master simulators you can use:
• ModScan (https://www.win-tech.com/)
• Simply Modbus (https://simplymodbus.ca/)
• Modbus Poll (https://www.modbustools.com/download.html)

 

>then I recommend trying a computer with Modbus simulator software and a USB to RS-485 adapter.

Me, too, because the Modbus simulator does the decoding so you read hex or decimal numbers to know what's going on, instead scope waveforms.