Dear programmers,

This is first time when I use Siemens forum, I'm totally new in TIA Portal 13 and my task is to poll 4 slaves on Modbus RTU, collecting the data, send them into a HMI (over ethernet) and represents PLC with Protection relays & HMI as a slave to a superior HMI (using second CM1241 module over RS485) . I'm using S7-1200 with CM1241 (RS485) in forst slot as a master and Argus 7SR210x (Siemens protection relays) as a slaves. For communicate with each slave I use MB_MASTER block (using MB_COMM_LOAD once before, in Main [OB1] with system bit "FirstScan" - REQ input). MB_COMM_LOAD passes OK (status 16#0000) but when I try to read every Argus protection in loop, sometimes I get 16#8200 status (which means "Port is bussy processing a transmit request" as manual says). Since I stop program flow with "Steps" (and theese steps "hold" reading/writing to the next slave until read/write operation of actual slave is done - with or without errors (in case of error I set error bit)), I have no idea where the 16#8200 status came from. 

Would anyone of you be so kind and give me an advice? And if you take a look at the program, I think It could be write much better (using a FC or somethnig like this) so would you mind give me any advices in that way?  Thank you for you ideas. I appreciate your kindness.

Martin