Modbus 485 RTU and bias resistance with DIGI IAP

Dear Forum;
I have a softstarter with a comm card.
This is a half duplex port.Voir attachment.

I set my DIGI IAP in order to have Modbus TCP in the other side. I have some communications but 20% errors. Coomunication is not reliable. I spent a lot of time investigating and I read something about bias resistor.

Could someone help me with some tips?

The Digi One IAP (in fact most Digi box products) ONLY have a bias on RS-485 when you enable our terminator. So if the DOIAP has the terminator DIP switched off, you might have a floating RS-485 line anytime the DOIAP is listening - which might be most of the time. When floating, the RS-485 line is a wonderful noise antennae feeding false 0xFF data bytes into both ends.

Since the Benshaw states it doesn’t have bias, you’ll need to enable the Digi One IAP’s terminator, which enables our build-in line bias.

If you cannot use the terminator, then you need to add perhaps an external pair. A common example might be 1000 ohms pull-down on the “-” lines to the signal ground and 1000 ohms pull-up on the “+” lines to something in the range of 4 to 8vdc. The level isn’t so important, but don’t violate any grounding or isolation designs to find the +voltage.