Acceleport 16/EM

I have 16 port XEM (S/N V73734519) that is not communicating Modbus information. Very similiar to,4231#13216
When I use the system’s COM1 everything works, but switching to the Digi COM5 (first port) or COM6 does not work. I even installed a USB to Serial convertor (COM3) and it works fine, so am very certain the problem lies with the Digi. I can use Baseterm to successfully communicate thru the Digi ports to one another, so am stumped as to why the Modbus traffic is having/causing an issue, or what the problem may be.