Testing BACnet MS/TP Devices with Visual Test Shell
CAS BACnet Explorer:
Available as a free tool for discovering, browsing, and testing BACnet devices.
This article provides step-by-step procedural guidance on using the Visual Test Shell (VTS) to test BACnet MS/TP devices. It covers configuring tests, interpreting results, and validating device behavior on an MS/TP network.
For general information about the Visual Test Shell user interface and capabilities, see Visual Test Shell (VTS): Overview & Features.
BACnet MS/TP (Master-Slave/Token-Passing) devices communicate over an RS-485 serial network and are commonly used in building automation systems for controllers, sensors, and field-level devices. Testing and validating MS/TP devices requires tools that understand BACnet services and the timing behavior of the MS/TP data link layer.
The BACnet Visual Test Shell (VTS) is a diagnostic and conformance testing tool designed to generate, transmit, and analyze BACnet messages. While VTS is frequently used for BACnet/IP testing, additional considerations apply when the target device resides on an MS/TP network rather than an IP network.
This article explains the supported methods for testing BACnet MS/TP devices using VTS, the required network topology, and the limitations that engineers should understand before attempting communication.
Why VTS Cannot Connect Directly to MS/TP via RS-485
VTS does not support direct communication over a PC’s RS-485 serial port for MS/TP traffic. Although MS/TP is defined over RS-485 at the physical layer, correct operation requires precise token timing, frame handling, and state management that are not implemented through a generic serial interface in VTS.
As a result, connecting a USB-to-RS-485 or onboard serial adapter directly to an MS/TP trunk will not allow VTS to participate correctly as an MS/TP node. The tool is not designed to act as a native MS/TP master or slave using standard PC serial hardware.
Common Architecture for Testing MS/TP with VTS
The most common and supported approach is to run VTS on a BACnet/IP network and communicate with the MS/TP device through a BACnet IP-to-MS/TP router. In this architecture, VTS generates BACnet/IP messages, which are routed and translated into MS/TP frames by the router.
From VTS’s perspective, the MS/TP device appears as a standard BACnet device reachable over IP. The router handles all MS/TP timing, token passing, and physical layer behavior, allowing VTS to focus on BACnet application and network layer services.
Using the MS/TP Port Option in VTS (NB-Link)
VTS includes an MS/TP configuration tab in its Ports dialog, but this option is specific to a hardware interface known as the NB-Link. The NB-Link is a dedicated MS/TP interface device manufactured by :contentReference[oaicite:0]{index=0}.
When an NB-Link is present, VTS can use it to participate directly on an MS/TP network. Without this hardware, the MS/TP tab in the VTS Ports dialog cannot be used. No generic RS-485 adapter can substitute for the NB-Link in this context.
IP-to-MS/TP Router as the Required Alternative
If an NB-Link is not available, an IP-to-MS/TP router is required to test MS/TP devices using VTS. The router must be properly configured with the correct MS/TP parameters, including baud rate, MAC address range, max master value, and network numbers.
Once configured, the router exposes the MS/TP network to the IP side, allowing VTS to discover devices, send BACnet services, and receive responses as if all devices were native BACnet/IP nodes.
Typical Test Workflow Using VTS and an IP-to-MS/TP Router
A typical workflow begins by connecting the PC running VTS to the same IP network as the BACnet IP-to-MS/TP router. The router is then connected to the physical MS/TP trunk containing the target device.
After configuring the VTS IP port, engineers can perform standard BACnet operations such as device discovery (Who-Is / I-Am), property reads and writes, and service-level testing. The router transparently forwards these requests to the MS/TP device and returns responses to VTS.
This approach allows validation of object models, property behavior, and protocol compliance without requiring VTS to directly manage MS/TP token passing or serial timing.
FAQ
Can VTS act as a native MS/TP device?
Only when used with the NB-Link hardware. VTS cannot natively act as an MS/TP node using a standard PC
RS-485 interface.
Is a BACnet router required for MS/TP testing?
Yes, unless an NB-Link is available. An IP-to-MS/TP router is the standard and most widely supported method.
Does testing through a router affect BACnet behavior?
The BACnet services and responses remain the same. The router only translates between BACnet/IP and MS/TP
at the network and data link layers.
Why is this approach common in real systems?
Many production systems already use routers to integrate MS/TP field networks with IP backbones, making
this testing architecture representative of real deployments.