Communication Network; Rs-485 Serial Link - Emerson 810-1600 Installation And Operation Manual

3.6. Communication Network

3.6.1. RS-485 Serial Link

The thermostat is equipped with a RS-485 serial data link to communicate with the
BMS. Communication with the Emerson Programmable Touchscreen Thermostat is
performed via the Modbus RTU protocol over the RS-485 link. The Modbus
network is used to read the status of the thermostat and/or apply new settings to
modify its operation.
There are three (3) switches (DIP switch) that allow enabling or disabling some 
termination and bias resistors for the proper function of the RS-485 network (see
Figure 3-27). These switches are located on top-right corner when the thermostat
enclosure is open.
Figure 3-27 - Modbus RS-485 Network Configuration Switches
Switch #1: Pull-up resistor enable (PU+)
This switch provides the ability to add a pull-up resistor (510 Ohms) on the RS-485
network for bias purposes. Normally, this switch is in the OFF position (no pull-up
resistor on the network).
Switch #2: End of Line resistor enable (EOL)
This switch provides the ability to add a termination resistor (120 Ohms) on the 
RS-485 network for termination purposes. If you do have a termination resistor 
elsewhere on your network setup, put this switch in the OFF position. If you do not
need a termination resistor from the thermostat, put this switch in the ON position.
Switch #3: Pull-down resistor enable (PD-)
This switch provides the ability to add a pull-down resistor (510 Ohms) on the 
RS-485 network for bias purposes. Normally, this switch is in the OFF position 
(no pull-down resistor on the network.)
26 • Emerson Programmable Touchscreen Thermostat I&O Manual 026-1739 Rev 0