Hi,

I have to communicate with several devices (clients) with my S7-400.

On some of them, I have both write and read functions.

My first question is, can I do reading/writing at the same time for all different clients?

Second, I suppose that using the write and read function on the same client can not be done at the same time? Can you give me some advice on how to "split" these two functions?

Should I, for example, use the positive edge of 1-second pulse for ENQ on writing, and the same negative edge for ENQ of the reading block?

Thank you.

Wüstenräuber

Hi,

first: When you want to read/write values from different devices, then the S7 CPU is the modbus client (active partner) and the devices are the modbus servers (passive partner).

With the same modbus block you can send only 1 telegram to the device at the same time. You must wait until the response is received, then you can send the next telegram.

For multiple severs you must use multiple modbus blocks. Each connection must use 1 unique modbus block. (If you use MODBUSPN, then you could use 1 modbus block, but then you must establish the connection to server 1, send telegram, disconnect, connect to server 2, send telegram.... It takes much time.)
For reading and writing telegrams you can use the same modbus block for 1 server, but you must start the telegrams one after another. If you want to send reading and writing telegrams parallel to the same server, then you must use 2 connections and 2 modbus block instances for 1 server.

Have a look at the manual or the block online help for the explanation of ENQ: With a positive edge of ENQ you start a telegram (reading or writing).

Wüstenräuber

Actually, I wrote wrong, but I have done that (I'm the client). Thank you for the detailed explanation. Before I asked a question, I have prepared one MODBUSPN block for each server, and also each for writing and for reading (4 servers, 8 MODBUSPN blocks). I just wasn't sure if they can work all at the same time in "parallel", but after your explanation, I think everything is clear to me now.

Just one more question, when it is about the same server, do I use different network ID's for writing and reading?

Thank you again for all your help.

Hello,
I know this post is old, but I have a question about this issue.
As I read in the comments, an S7 CPU acting as a client can only send one telegram to a server at a time. So in the case of multiple servers, at any given time, can the client simultaneously send telegrams to those servers, ensuring that each server only receives one telegram at a time?