www.qualitrolcorp.com
Appendix G
–
Modbus Communications
30
Appendix G – Modbus Communications
If the Model 118 ITM is equipped with the optional RS-485 port, the unit can communicate
with a Modbus master station.
The RS-485 port is located on the side of the Model 118 ITM and is provided with a plug-in
terminal block connector. The connector is wired as follows:
Terminal – TB4
Function
1 +Receive
2
-
Receive
3 +Transmit
4
-
Transmit
5 Shield
Ground
Terminals 1 and 3 and terminals 2 and 4 can be connected together if a two-wire
RS-485 data link is desired.
The data communicated on the RS-485 link is in Modbus RTU format. The table included
shows the data and Modbus register addresses used. Modbus functions 03, 06, and 16 are
supported.
The Modbus Data Link parameters can either be set using the User Setup software or they
can be set up by the following procedure using the front panel switches:
1. Depress and hold all three switches until the unit stops cycling through the readings
(approximately 10 seconds). The unit will then be displaying the current baud rate:
12 = 1200 bits per second (bps)
24 = 2400 bps
48 = 4800 bps
96 = 9600 bps
192 = 19200 bps
Pressing the ALARM (Top) button will step the display UP through the above
choices. Pushing the FAN (bottom) button will step the display DOWN through the
above choices.
2. When the desired baud rate is displayed, press the MAX TEMP RESET (Middle)
button. This will show the current parity setting.
PE = Even Parity (standard)
PO = Odd Parity
P-- = No Parity
3. When the desired parity is displayed, press the MAX TEMP RESET (Middle) button.
This will show the current stop-bit setting. If parity is even or odd, the display will
show S1 and will not be able to be changed.
