Configuration of the MS/TP Supplement

MS/TP Introduction

General

MS/TP (Master Slave / Token Passing) specifies connections between BACnet devices using the standard EIA-485 for serial networks. Typical devices using MS/TP include VFD=variable frequency drives, pumps, actuators, room-operating units, sensors or actuators as well as compact air-conditioning units or chillers..

Layer 1 EIA-485

MS/TP networks use an EIA-485 connection. The following requirements shall be fulfilled:

  • Twisted Pair cable AWG18 or better(0,82mm² conductor area of the signal wires) with foil or braided shield
  • Impedance 100-130 Ohms
  • Capacitance <100pF/m (30pF/ft) between conductors
  • Capacitance <200pF/m (60pF/ft) between conductors and shield
  • Max. (recommended) length of a single segment is 1.200m (4.000ft) with AWG18 cable (0,82mm² conductor area). Depending on the number of connections the actual maximum length of a segment may reduce to 500-700m only. Due to this the maximum specified segment length for the terminal EL6861 is limited to 500m.
  • Max. number of devices per segment: 32 (using the terminal EL6861 and devices w/ low-power transceiver: 64 devices per segment)
  • Polarity is important, the signal wires are marked: B+, A- (Attention, sometimes devices are marked with an incorrect A+, B-, in this case the signal wires must be swapped.
  • To connect segments, segment-repeaters may be used.
  • The wiring is specified to use Daisy-Chain only, starting with the first device directly from device to device. Using T-Junctions or star topology is not allowed.
  • To prevent from ground-loops, the shield shall only be connected to potential ground at a single point. The BACnet standard ANSI/ASHRAE 135 specifies various deployment options for various installations.
  • The connector is not specified. The Terminal EL6861 uses a Profibus-compliant 9-pin Sub-D connector.
  • The serial network shall be terminated at both ends (default: 120 ohms, tolerance: 5% between the signal wires).
  • A minimum of 1, maximum of two network bias resistors shall be installed in every MS/TP network (default: 510 ohms, tolerance: 5%), preferably at the ends of the network. These resistors keep the signal wires at pre-defined signal levels and allow for a better signal detection of the transceivers.
  • Termination and network-bias using the EL6861: For this terminal the values specified in the Profibus-standard have been implemented: Termination (220 Ohm) and network-bias (620 Ohm) using the optional Profibus-connector. Network-Bias and termination may optionally be switched using the built-in DIP-switch of the Profibus-connector.
  • An isolation of at least 1500V is required if connecting devices between buildings. This may be installed using optional segment repeaters w/ built-in isolation

Logical Layer 2

The MS/TP protocol on ISO/OSI layer 2was exclusively specified for use with BACnet. MS/TP specifies a multi-master / multi-slave serial network. Due to potential collisions only one device is allowed to send to the network simultaneously. The right to send is transferred using a token, which is passed from device to device. After sending to the network a device is obligated to distribute the token top the successor device. To determine the successor all MS/TP devices periodically send a Poll-For-Master request. A positive response identifies the successor device. With this new devices may enter the token-ring or devices not being present anymore are detected and will be excluded from the token-passing.

Master devices are active network participants and may request data from other master- or slave devices (e.g. to read values or to command set-points by writing to others).
Slave devices are –by nature- passive and are not allowed to access the network, except they are requested by a master device. In this case they are obligated to respond to the request.
Dynamic Device Binding (DM-DDB-A) using a BACnet-broadcast message “Who-Is” cannot be used by slave devices.

The majority of MS/TP devices are master implementations though. Some devices may be configured in both master- and slave mode. Devices which only support slave-mode are rare.

Interface settings

MS/TP uses NRZ (non-return-to-zero), the interface settings are fixed:

  • 1 Startbit
  • 8 Databits
  • 1 Stopbit
  • No Parity
  • Short form: 8,N,1

Normally these settings are fixed within MS/TP devices and don’t need to be modified. Some devices (VFD, pumps, actuators or VAV-boxes), especially those supporting other protocols like Modbus as well, may require to setup the interface parameters as shown above.

All devices within the network shall be configured to use the same speed. Like always for serial communication, the speed is specified in “baud” (baud described the number of characters transmitted per second).

The following baud-rates are specified for BACnet MS/TP:

  • 9600 baud*
  • 19200 baud
  • 38400 baud*
  • 57600 baud
  • 76800 baud
  • 115200 baud

The baud-rates marked with * are required to be supported by all MS/TP devices.

The terminal EL6861 supports all baud-rates specified in the BACnet-standard as shown above.

Device addressing

Every device in a MS/TP network (master or slave) shall be configured using an individual address (MAC address=Media Access Control). The address length is 1 octet and is typically setup using a software tool or DIP-switches. The address shall be unique within the MS’/TP network.

Address 0-127 may be used for master devices, where 0-254 may be used for slave devices.

Address 255 specifies the broadcast address (telegrams sent to all devices on the network) and shall not be used as source address. The max. length of telegrams in MS/TP is specified to 501 octet (21 octet network layer header plus 480 octet application layer data).

Specific characteristics when communicating w/ MS/TP devices

The benefit of MS/TP is cheap wiring using long distance connections. But this results in lower transmission speed compared to Ethernet networks. In addition the BACnet implementations of MS/TP devices are often very limited. The majority of devices conforms to the BACnet device profile B-ASC (BACnet Application Specific Controller), not supporting functionality like COV (change of value notifications) as well as ReadPropertyMultiple (reading multiple properties at once). This means communicating to MS/TP devices may require a lot higher reply-times than using Ethernet/IP-connections, especially when requesting larger data portions from devices.

Due to this it is highly recommended to poll only those properties which are required in the application or user representation. Static properties may be requested only once or upon request after it is known that they have changed.

It is recommended to adjust the poll-rates to higher cycle-times to not overload smaller MS/TP devices w/ less resources compared to PLC-controllers.

Routing between MS/TP and BACnet/IP

A device may optionally provide routing capabilities between MS/TP and BACnet/IP. Along with the supplement TS8020 (BACnet/IP) the terminal EL6861 may be optionally configured as a BACnet router. Using multiple EL6861 terminals may setup a multi-channel BACnet-router.

Optimizing the communication

Assigning more bus-access

MS/TP devices which require more access to the network compared to other devices (like BACnet routers to connect different networks) may be assigned a higher number of frames than other devices. The setting „Max_Info_Frames“ in the device object specifies the number of frames the device is allow to send before it has to pass the token. The default value for the terminal EL6861 is specified for 20 Infoframes because the PLC usually requires more bandwidth on the network compared to the devices connected. The default value for all other devices on the network is suggested to equal 1 Infoframe.

Specifying the highest station address

Cyclic „PollForMaster“ telegrams to determine successor devices may cause a heavy bandwidth usage on the network if station addresses are not carefully configured.

Every station address higher than the own station address is required to be requested which leads to unnecessary PollForMaster requests in case of gaps in the addressing. In a perfectly configured network the first device is configured using MAC-address=0 (default value for the terminal EL6861). Further devices should be addressed in ascending order starting with 1, 2, 3 etc. The highest station address used within the network may then be specified in the property MAX_MASTER in the device object of all devices.

Hint: In case some devices do not support writable MAX_MASTER properties (in this case the value is required to equal 127), the terminal EL6861 may be configured using the highest known station address and this setting may be configured in the MAX_MASTER setting of the EL6861 MS/TP configuration.