Thank you

Duris, Cake,Mop for support.

Now it's working fine.

Dear Forum Members,

I have spent days at site related to Modbus RTU communication as - CM PtP connected to RIO - is master and 3rd party devices are slaves.

I have seen several incomplete forum topics on issues, troubles and problems.

I would like to summarize my experience below.

Golden Rule 1:It is required to run Comm_Load properly at first. You might think there is no existing error, however since REQ is asked at the first scan, an error could have occurred. So error code should be moved to any area to get “inspired”. Idle status can also confuse your mind. Ultimate goal should be getting no error or no idle status from Comm_Load.

Reminder 1:To my experience, you don’t need to call Comm_Load in OB1 or OB100 if there is no other factor restricting you in your program such as cycle time issues, etc.

Reminder 2:Error codes should be considered as a general description. You can mis-wire Modbus cable, in this case you will face with a port fault.

Reminder 3:To not to add any master or slave block will not prevent you from running Load_Comm properly. So my suggestion is to insert Comm_Load at first and execute it properly and independently.

Golden Rule 2:Monitor DB of Comm_Load variables and compare online-offline columns such as stop bits, etc. This will “inspire” you. As a first sight, it can be assumed that correct hardware configuration parameters plus correct Comm_Load block input values would be adequate. However, it is possible that you might need to interfere start values column at Comm_Load column. Such as forum recommendations of:

DB / Static / Mode = 4

Or

DB / Static / Line_Pre = 2

depending on you application.

Comment column should be inspected carefully since some parameters are for internal use such as “port_w_record_header”.

Reminder 4:In many threads I have seen suspicious entries if there is a difference between firmwares of Comm_Load system blocks. To my experience there is no worry on that. But there are some differences between 1200 and 1500 CPU’s. In a Siemens documentation I have seen that there was a correction in 01.2018 related to Comm_Load. See attached picture pls.

Golden Rule 3:The same approach should be implemented for master or slave blocks as well but just after you get no error or idle status from Comm_Load.

Warm regards.