Are there any limitations to expansion units on GSM PLCs?

Yes – If you are using expansion units (EXM-E-) with your GSM CPU then you cannot use EXM-E-RS485 as well.
If you require RS485 on your GSM CPU system, then please choose the PRO-RS485, which plugs into the programming port of the PLC CPU.
If you are not using ANY other expansion units with your GSM PLC CPU and need RS485 connectivity, then you can use the EXM-E-RS485 module.

An EXM CPU can have up to a maximum of 8 expansion units.