INTRODUCTION
This document describes the serial communications protocol of the HAI Omnistat2 RC1000 and RC2000
communicating thermostats. The Omnistat2 Series thermostats are capable of several communications modes.
This document describes two: RS-232 mode and System mode. These differ in electrical connections and
transmission speed. The messages, however, are the same for both modes.
ELECTRICAL CONNECTIONS
RS-232 mode
Intended for connection to personal computers or other systems with RS-232 ports. 4 wires to the
thermostat: DTR, RXD, TXD and Ground. All wires are optically isolated in the thermostat. The
thermostat transmits with a modified RS-232 transmitter that can be tri-state (high impedance) so that
more than one thermostat can be connected in parallel to a single RS-232 port.
In RS-232 mode, the transmitted RS-232 voltage levels are generated by using DTR as a source for +12
volts when transmitting a space (0 bit) and the received RS-232 signal as a source for -12 volts when
transmitting a mark (1 bit). Therefore, DTR must be set high at all times by the host, and the host must
not transmit while a thermostat is transmitting.
A typical RS-232 port on a personal computer can drive 4 thermostats directly using HAI p/n RC-201
cable. A signal booster (HAI p/n RC-202) is needed to increase the output current of the RS-232 port
for more than 4 thermostats.
System mode
Intended for connection to automation systems that do not have RS-232 voltage levels. 3 wires to the
thermostat: TXD, RXD, and GND. The communications wires are optically isolated from the HVAC
circuits in the thermostat. RXD is 0V when the host is sending a mark (bit 1) or idle, 12 V when host is
transmitting a space (bit 0). The thermostat TXD is an open collector optoisolator, open when idle or
transmitting a mark (bit 1) or conducting to GND when transmitting a space (bit 0.)
ELECTRICAL SPECIFICATIONS
RS-232 mode:
Thermostat receiver - The thermostat will draw the following current at the following voltage:
RS-232 receive space (bit 0):
2.0mA at 12V
RS-232 receive mark (bit 1)
transmitter idle:
0.0mA at -12V
transmitter active:
-4 ma max at -12 V
Thermostat transmitter - The thermostat will provide the following current and voltage. Note that +12 is
drawn from DTR, -12 V is drawn from the thermostat receive line (TXD) while it is idle.
tri state: (off):
open collector, 0mA
RS-232 transmit space (bit 0):
10V at 4mA
RS-232 transmit mark (bit 1):
10V at -4mA
Note: these levels are compatible with RS-232.
Copyright © 2008 Home Automation, Inc.
All Rights Reserved
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Summary of Contents for Omnistat2 RC-1000
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