Hello,

I'll be start tests with lower comm speed - 9600. In this case you have on Gateway MB TCP side some PC application or some S7 PLC HW? In case that both devices are correctly set-up (that Gateway is very important because change one protocol to other with different media), you need connect via PC Client application to gateway as MB TCP Server. Then it make other business without your intervation. What you did till this time?

Update: there are also other PC applications simulated MB TCP Client, look to web. My preffered:

www.simplymodbus.ca

Hello,

in my opinion you can better now test your relay via USB-RS485 converter or other suitable converter. Realy sry, I don't understand your sitaution with very hard communication, simply try for first connect to relay with it's medium - RS485. You can test all requests, collect data from Slave via serial line. After you'll have success, you can continue with that Gateway. You'll know that your RTU device is working properly via RS-485 medium and next configuration/test via TCP protocol will be simplest as your now situation. You'll then know that if some problem occur, it can depends to that Gateway cfg. Look also to Gateway documentation about termination, where and how it must be done. This is only my idea, also how will I solve this situation in your place.