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TC300 Modbus bias, termination and number of devices

Issue

What is the unit load of a TC300 thermostat and what are the bias and termination requirements when used with a SmartX Server.

Product Line

EcoStruxure Building Operation, Field Devices

Environment

  • TC303-3A2LM
  • TC303-3A4LM
  • TC303-3A2DLMS
  • TC303-3A4DLMS
  • SmartX Server

Cause

Documentation does not state the RS-485 transceiver type for the TC300 range, making it unclear how many devices can exist under a SmartX servers RS-485 serial port ComA or ComB.

Resolution

The answers to the transceiver interface attributes below can be applied with to the Worksheet for Configuration of RS-485 Bus with generic RS-485 devices on Webhelp

 

Q1 Failsafe Receiver

The TC300 does not use an RS-485 transceiver with an integrated idle-state failsafe receiver.

 

Q2 Data Transmission Speed

TC300 thermostats manufactured from January 2017 onwards can be configured to communicate at 9600 or 4800 bps. TC-300 thermostats manufactured before this date can only communicate at 4800 bps.

 
Q4 Isolated Interface

The TC300 does not provide an isolated RS-485 interface.

 

Q6a Measured Resistance

With no electrical connections to the TC300, the measured resistance between the RS-485 low-side (-) signal and the communications common is 10K ohms. Based on this the unit load of the TC300 is 1.2 (12.000/10,000)

 

Bias requirements and number of devices

If the length of the RS-485 bus is less than 150M (500ft) then Generic RS-485 Device Configuration 3 from Webhelp can be applied. Using this configuration the recommended 3300 bias resistors present a unit load of 3.6 (12,000 / 3,300). The SmartX server adds 0.5UL. The remaining node budget is calculated as: 32 - 3.6 - 0.5 = 27.9UL. Since the TC300 has a unit load of 1.2 the maximum number of devices that can be placed under each comm port using this configuration is 27.9 / 1.2 = 23.25

 

If the length of the RS-485 bus is greater than 150M (500ft) then Generic RS-485 Device Configuration 7 from Webhelp can be applied. Using this configuration the dual end-point bias configuration presents a unit load of 24UL (12,000 / (1,000 / 2) = 24). The SmartX server adds 0.5UL. The remaining node budget is calculated as: 32 - 24 - 0.5 = 7.5UL. Since the TC300 has a unit load of 1.2 the maximum number of devices that can be placed under each comm port using this configuration is 7.5 / 1.2 = 6.25

 

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

 

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