In order to test a BACnet MSTP device using the Visual Test Shell (VTS) there are a few items to take into consideration.
1) The VTS will not connect to an MSTP link using an RS-485 port on a PC.
2) The most common way to connect VTS with MSTP is to connect the VTS on IP and use a BACnet IP-to-MSTP router.
3) The MSTP tab in the VTS Ports dialogue uses a device called the NB-Link from American Auto-Matrix.
If you do not have a NB-Link from American Auto-Matrix you must use the IP-to-MSTP router option to test the MSTP device with VTS.