How Does BACnet MS/TP Discover New Manager Devices?
BACnet MS/TP uses token passing to control which devices are allowed to initiate communication on an RS-485 network. Manager devices participate in the token ring and may initiate communication when they hold the token. Subordinate devices do not participate in token passing and respond only when requested.
When a new MS/TP manager device is added to an operating network, the existing token ring must discover it before the new device can participate in token passing. BACnet MS/TP provides a specific mechanism for this process called Poll For Manager.
Older BACnet documentation and many existing products use the terms Poll For Master and Max Master. Current BACnet terminology uses Poll For Manager and Max Manager. The underlying MS/TP function is the same.
The BACnet MS/TP Token Ring
Each participating manager device has a unique MS/TP MAC address. Manager addresses are in the range 0 through 127.
During normal operation, the token moves from one manager device to the next. A manager that receives the token may initiate BACnet communication and then passes the token to the next known manager in the ring.
The network therefore needs a way to determine whether a new manager has appeared at a MAC address that is not currently part of the token rotation. This is the purpose of the Poll For Manager process.
Learn more: What Is BACnet MS/TP?
What Is a Poll For Manager Message?
A Poll For Manager frame is an MS/TP control message used by manager nodes to determine whether another manager exists at a particular MAC address.
If a manager device receives a Poll For Manager addressed to its MAC address, it responds with a Reply To Poll For Manager frame. That response tells the polling device that the responding manager is present and wants to participate in the token ring.
The responding manager can then become the polling device's successor in the token rotation, allowing the token to be passed to it during normal operation.
Subordinate devices do not join the token ring and do not respond as manager devices to this polling process.
How the Network Searches for New Managers
An MS/TP manager keeps track of the next known manager to which it should pass the token. During token-ring maintenance, unused MAC addresses can be checked to determine whether another manager has appeared between the current device and its known successor.
For example, suppose manager address 1 normally passes the token to manager address 5. Addresses 2, 3, and 4 are potential locations where a new manager could be added. The Poll For Manager process allows those addresses to be checked so that a new manager can be added to the token rotation.
If no manager responds at the address being checked, normal token passing continues. The maintenance process progresses through the eligible address range over subsequent polling cycles.
How Often Does BACnet MS/TP Search for New Managers?
BACnet defines a parameter called Npoll that controls periodic maintenance of the token ring. The standard value is 50 token uses or receptions before a Poll For Manager maintenance cycle is performed when there are addresses that need to be checked.
This prevents the network from constantly polling unused addresses. Polling is necessary so that new manager devices can join the ring, but every polling frame also consumes time on the serial network.
The result is a balance between discovering newly connected managers and minimizing unnecessary MS/TP overhead.
What Is Max Manager?
BACnet MS/TP also uses a parameter called Max Manager. Older equipment and documentation commonly call this parameter Max Master.
Max Manager specifies the highest MAC address that the manager should consider when searching for other manager devices. Its maximum permitted value is 127.
For example, if a device has Max Manager set to 50, it will not search MAC addresses above 50 when performing normal token-ring maintenance. A new manager configured with MAC address 60 therefore cannot be discovered through polling performed by a device whose search range stops at 50.
This is an important configuration item when adding devices to an existing MS/TP network.
Why Not Always Set Max Manager to 127?
A Max Manager value of 127 allows the full manager-address range to be searched and provides room for additional devices. However, unused addresses may still need to be checked during token-ring maintenance.
On a network where the highest manager MAC address is known, configuring an appropriate Max Manager value can reduce unnecessary polling of addresses that will never contain a manager.
The value must not be set below the MAC address of any manager that needs to participate in the network. When future expansion is expected, the configuration should also leave enough address space for the additional manager devices.
Poll For Manager Is Not the Same as BACnet Device Discovery
One of the most important distinctions is that MS/TP token-ring discovery is not the same as BACnet application-layer device discovery.
Poll For Manager operates at the MS/TP data-link layer. Its purpose is to locate manager nodes and maintain the token ring.
Who-Is and I-Am are BACnet application-layer services used to discover BACnet devices. A BACnet application can send a Who-Is request, and matching devices can respond with I-Am messages identifying their BACnet Device instances.
This means an MS/TP MAC address and a BACnet Device Instance are different identifiers serving different purposes.
MAC Address vs. BACnet Device Instance
The MS/TP MAC address identifies a node on the local MS/TP data-link network and is used for functions such as token passing.
The BACnet Device Instance identifies the BACnet Device object at the application layer. BACnet Device Instance numbers are intended to uniquely identify BACnet devices within the BACnet internetwork.
A device therefore has both an MS/TP MAC address and a BACnet Device Instance. They are not required to have the same numerical value.
Duplicate MS/TP MAC addresses can disrupt token passing because two devices attempt to use the same data-link address. Duplicate BACnet Device Instances cause a different type of addressing conflict at the BACnet application layer.
Learn more: What Happens When Two BACnet MS/TP Devices Have the Same Device ID?
Why a New BACnet MS/TP Manager May Not Join the Token Ring
If a newly installed manager device does not appear to participate in the MS/TP network, check the data-link configuration before assuming that BACnet device discovery is the problem.
Common causes include:
- The new manager's MAC address is higher than the configured Max Manager search range.
- Two devices have been configured with the same MS/TP MAC address.
- The new device is using a different baud rate from the rest of the MS/TP segment.
- RS-485 polarity, termination, cabling, or other physical-layer problems are preventing reliable communication.
- The device is configured as a subordinate rather than as a manager capable of participating in token passing.
These conditions should be checked separately from BACnet Who-Is/I-Am application discovery.
Troubleshooting MS/TP Discovery
Monitoring MS/TP traffic can help determine whether Poll For Manager, Reply To Poll For Manager, and Token frames are being exchanged correctly. It can also help identify duplicate addresses, missing devices, excessive polling, and physical communication errors.
The CAS BACnet Explorer can be used to discover and troubleshoot BACnet devices and networks.
For a broader MS/TP diagnostic procedure, see: Troubleshooting Guide for BACnet MS/TP Networks