Fixed/Auto/Forced baudrate for MSTP

MSTP baudrate is always painful for field technician. If the baudrate is wrong, device can’t join a MSTP bus.

Most devices have fixed baudrate. To modify the baudrate setting, technician have to physically access the device and change dip switches. Some devices support changing baudrate by BACnet service. but before that they should already have correctly baudrate setting for BACnet service to access it.

Some vendors implement auto baudrate, but introduce more problem than it solves. There are two types of auto baudrate mechanism:

  • Starting detection: The device detects and adopts baudrate on the bus when it starts. then never changes baudrate.
  • Dynamic detection: The device does same as starting detection type when it starts, but if it find there are error on bus for a predefined time, it considers that the baudrate is changed, it detects baudrate again.

For both types, it is difficult to change baudrate when devices is working. Simply changing baudrate on all fixed baudrate devices can not work, because auto baudrate devices are still working on old baudrate. The solution is to power off all auto baudrate devices, then power on all auto baudrate devices(Don’t power off/on auto baudrate devices one by one)

Our new firmware(>=2.0) introduces new baudrate management mechanism(Patent pending). There are 3 types of baudrate mode for BACRouter : Fixed/Auto/Force:

  • Fixed baudrate mode works as most traditional devices.
  • Auto baudrate mode is same as above-mentioned dynamic detection. The predefined time to re-initiate detection is 10 error frames.
  • Forced baudrate mode is same as auto mode except that when the device get token, it changes baudrate to predefined value.

When there is a device with forced baudrate mode, the baudrate on the bus will be forced to predefined value. Devices with auto baudrate mode will automatically synchronize to predefined baudrate. Devices with fixed baudrate mode but baudrate setting different with predefined value will not be seen on bus (It’s easy to check out in “Recent active devices” field from BACRouter’s runtime info). Devices with starting detection type may run on wrong baudrate, they will not be seen on bus too, but powering off/on them one by one will synchronize them to forced value.

More than one device with forced baudrate mode could coexist on a bus, but the baudrate values on them should be same.