Hello,

I'm using a CM 1241 as a Modbus Master RTU.  As a new build, just trying to get it working with my laptop at the moment which is running a ''Modbus Slave'' program.

The ''MB_COMM_LOAD_DB'' is working fine and is ''done'' which allows the ''MB_MASTER_DB'' to start its request.  

However, its constantly stuck on BUSY.  I have added counters to the DONE and ERROR bit, both these stay at zero, the STATUS word is always at 0000 (no error).  

Monitoring the data coming out of the CM 1241 is as follows:  [01] [03] [00] [ff] - which translates to slave address 1, function code 03, start address 0 (or 40,0001) then the rest appears to be garbage as missing part of the instruction to the slave.  

It looks as if the MASTER has not finished its request correctly to allow the SLAVE to respond.  

I have tried multiple things including timings as thought was a timing issue.

Any ideas please?

Hello;

One important information on MB_Master (ad other modbus functions for TIA Portal) is that the Error and Status outputs of the block last only for one scan, and therefore they are not visible for long enough for you to catch them. Ths quote from our frien navnag should make things clearer:

"The error code value at theSTATUS parameter is valid only during the single scan where ERROR = TRUE. ( Easily missed by user if not stored )".

Look at the last post on thos page:

https://support.industry.siemens.com/tf/ww/en/posts/modbus-limitation/108957/?page=0&pageSize=10

So when you call your MB-Master block you must set a trap to store the value of the status error code, when the output Error becomes true.

Then you can examine the error code in the TIA Portal system manual, for example, and discover what went wrong.

Hope this helps,

Daniel Chartier

Hi Daniel,

Thanks for info.

Yes, created a capture of the error message and was saying ''16#8280 - Negative acknowledgment when reading module''

I then found this snippet if info ''In addition, you must change the static parameter MODE in the instance data of the "Modbus_Comm_Load" for the duplex operating mode, preferably via the start value (0 = full duplex (RS232), 1 = full duplex (RS422) four-wire mode, 4 = half duplex (RS485) two-wire mode).'

After changing it to a 4 (as I am using RS485 2 wire) it still didn't work!

Finally, after about to give up for the night, my 7 year old son entered my workshop like a wrecking ball, knocked my laptop that was simulating the ModBus Slave and it started to work!  So, I assume the fault was with a bad connection on my USB to RS485 converter.

Thanks.

 Hello;

You have just demontrated 2 very great truths in this world:

1. God seems to have a very bizarre sense of humor, it will drive us al crazy!

2. Thank god for our children, they might keep us sane...

Be sure to allow him access to your workshop whenever he feels like it!

Best regards,

Daniel Chartier

AutoGen_3128587

Hi Daniel,

Finally, after about to give up for the night, my 7 year old son entered my workshop like a wrecking ball, knocked my laptop that was simulating the ModBus Slave and it started to work!  So, I assume the fault was with a bad connection on my USB to RS485 converter.

Thanks.