Hello everybody!

I'm facing a problem regarding reading some modbus registers.I'm using the S7314C-2 PLC with a CP340-RS422/485 card. Although I can read a specific data area of some modbus addresses without a problem ( 3001 - 3100 to be exact), I can't read anything above this address (eg 7443) even if I try to read just one register. The device just doesn't respond. If we connect directly on the modbus device using a modscan software we can read these registers without a problem. The function code is 03.

Any ideas?

Hello,

for first pls, what Modbus solution you have use? Own? 3-rd party? Simply I see CP340 attached which doesn't support Modbus.

Duris_I

Hello,

for first pls, what Modbus solution you have use? Own? 3-rd party? Simply I see CP340 attached which doesn't support Modbus.

Hello!

Do you mean the programming code? We use a non Siemens solution that we have used for many years and for various devices without any problem whatsoever. It is currently used to read registers from another device also without a problem

Hello,

again pls, if you're using your own or 3-rd party Modbus driver solution, very sry. We can give you help only to Siemens driver in this case of Modbus. Look, how we can know that driver functionality? I can give you ideas but for first you must provide more infos about that Modbus driver, without to know more infos is hard to help. You can maybe attach here clear info

1. what Slave type device you poll now?

2. what FC code you run now with trouble, which other parameters of request? Attach here message frame in hex format f.e.x

01 03 ...

and answer from Slave with same format.

To more infos, also you can zip more pictures and attach zip file here, more helps from us.

Hello,

simply you need check outgoing frame from your CP340 (set in this case Modscan as Slave instead of real Slave, if it can work like that) and check incoming message <consistency>. I mean MB Slave address, FC code, address, how much data and CRC. I can recommend only this now.

Duris_I

Hello,

simply you need check outgoing frame from your CP340 (set in this case Modscan as Slave instead of real Slave, if it can work like that) and check incoming message <consistency>. I mean MB Slave address, FC code, address, how much data and CRC. I can recommend only this now.

 Thank you very much for your answers, I will look into them and I will let to know about the progress!

Hey,

if you're writing to a DB, try writing to a test DB, your values maybe being overwritten.

Cheers

Hello everyone!

We finally figured out what the problem was. For some reason the instrument returned double the size of registers than we asked for. For example, we were sending a request for 4 registers and we received 8! So our routine returned Error and didn't load the values from the source DB to the destination DB. And this only happens to the registers above 43100. Below that everything is normal. Has anyone seen that before?