Hi

I would like to ask for help because I have a strange problem which I need to get sorted as soon as possible. 1200 PLC with CM1241 RS485 module (Modbus RTU) communicates at 19.5kb speed with instrument which uses modbus ASCII protocol - there is modbus RTU to Modbus ASCII converter in between. At the beginning comms work fine and the PLC receives samples from the instrument every minute as it should but then (around 4-6 minutes) PLC generates an error "80C8" which suggests slave time out (error generated by modbus communication block). I started to monitor modbus traffic using hyperterminal to improve my troubleshooting as well.

I believe the instrument is fine as it was used in the other process but i cannot swap it for different.

What steps would you take to get to the bottom of this problem?

Before taking my time to analyse the program which i was told used to work 3 years ago I would like to find out if the comms settings are ok (it starts the sampling just fine so basic settings must be ok), I heard that maybe a time between frames is to short (which leads to problems later) - does anybody know how to change it? Can I use modbus simulator to feed the messages manually to the instrument (I have them written) to be 100 % sure that the instrument is fine?

Please share your thoughts if you can and many thanks to anybody for help.

Dear Mr Hewking,

do you mind to share the existing PLC program please?

for i have tried several times configuring the modbus block, but none of them are succesfully comunicate with the modbus slave.

thank you for your attention and help

Hi 

Thanks for your suggestion i will try it out when i am on site.

I noticed that when my PLC (modbus RTU master) sends request to the instrument to check its status if new sample is ready to be read, sometimes a slave does not reply to this request and the request has to be repeated. When new sample should be ready after every minute PLC keeps checking every second which i hope is not to often. 

Does modbus slave has to reply to every modbus master request if comms work properly or is it normal situation that replies sometimes does not arrive and the request has to be repeated?

Please share your expertise if you can. Many thanks

Hi 

I have one more piece in the puzzle which i am trying to solve. 

PLC retrieves the readings from the instrument every minute and the instruments seems to lock up and don't reply to the PLC request for several seconds when it is preparing new sample values (slow instrument and single threaded).

When this happens PLC keeps checking instrument status every 3 seconds but does not fail because it has 10 retries before failure.

I am monitoring the modbus traffic using hyperterminal and the strange thing is that during this lockup PLC sends a request to read the very first address in the modbus memory which is the  address 40 000 - this address is a request to read the version of the modbus memory map to which the instrument replies properly.

I could not find anywhere in the code an explicit request to read the address "40000 "  so is it possible that the modbus card itself is triggering this request?

Could you please help me to solve this strange situation?

Hi 

Thank you for the information. I checked the link and I understand the difference.

I double checked the PLC code and I don't trigger the read command of the holding register 40001 anywhere in the code. Also I added some diagnostic code trying to SET the coil when address is equal to 40001 but it never gets set but the strange read command is issued anyway from time to time.

I am not on site now and will try to use a modbus simulator trying to replicate the issue and watch the traffic in the hyperterminal.

Can you recommend any good modbus slave simulator?

Best Regards