TC900 Modbus bias, termination and number of devices

TC900 Modbus bias, termination and number of devices


What is the unit load of a TC900 thermostat and what are the bias and termination requirements when used with a SmartX Server.

Product Line

EcoStruxure Building Operation, Field Devices


  • TC907-3A4LMA
  • TC907-3A4DLMSA
  • TC907-4FMSA
  • TC907-3A4DPMSA
  • TC907-3A4DPMSA-24
  • TC903-3A4LMA
  • TC903-3A4DLMSA
  • TC903-4FMSA
  • TC903-3A4DPMSA
  • TC903-3A4DPMSA-24
  • SmartX Server


Documentation does not state the RS-485 transceiver type for the TC900 range, making it unclear how many devices can exist under a SmartX servers RS-485 serial port ComA or ComB.


Use the Worksheet for Configuration of RS-485 Bus with generic RS-485 devices on Webhelp to determine the appropriate Modbus configuration. The answers to the transceiver interface attributes below can be applied to complete the worksheet.

  • Q1 Failsafe Receiver : The TC900 uses an RS-485 transceiver with an integrated idle-state failsafe receiver.
  • Q2 Data Transmission Speed : TC900 thermostats can be configured to communicate at 4800, 9600, 19200 or 38400 bps.
  • Q4 Isolated Interface : The TC900 does not provide an isolated RS-485 interface.
  • Q6a Measured Resistance : With no electrical connections to the TC900, the measured resistance between the RS-485 low-side (-) signal and the communications common is 8.2K ohms. Based on this the unit load of the TC900 is 1.46 (12.000/8,200)

Bias requirements and number of devices
If there are only TC900 devices on the RS-485 bus then Generic RS-485 Network Device configuration 1 applies, configuration becomes very simple and no node or distance reductions are called for. The failsafe receivers can typically operate with no added bias resistors.

  • Connect a 120 ohm termination resistor across the + and - data lines at the head end of the bus (typically at the SmartX server). Connect another 120 ohm termination resistor across the + and - data lines on the last node at the far end of the bus.
  • Connect the shield drain wire to earth ground terminal rail in the panel with the SmartX server. This is the only ground connection of the shield for the complete cable segment. Connect the RET/Shield terminal (see the figure below) on the SmartX server to the ground rail in the panel using a 12 AWG (3.31 mm²) to 18 AWG (0.82 mm²) wire.
  • Since the TC900 has a unit load of 1.4 the maximum number of devices that can be placed under each comm port using this configuration is 21 (31.5 / 1.46 = 21.5)

Also check out the SmartStruxure-RS485-Controller Unit Load Quick-Help video on the Exchange.

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