PureAire Monitoring Systems, Inc.
10/25/07
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7: Appendix
7.1 Modbus RS-485 Serial Interface
The optional Modbus RS-485 serial interface board allows up to 128
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gas monitors to communicate to a Modbus master device — such as a PLC
or PC running MMI or GUI software equipped with a Modbus driver — on a
single cable. With this option, the Modbus master can interrogate each connected
slave device (i.e.,
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) and retrieve information made available in
specific register locations. It supports cable lengths up to 4000 feet and both 4-
wire full duplex and 2-wire half duplex connections.
7.1.1 RTU Address
The 8-position DIP switch on the Modbus RS-485 PCB allows different RTU
addresses to be assigned to each
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. The eight DIP switches
represent an 8 bit binary number with 1 = LSB and 8 = MSB.
EXAMPLE: OFF, ON, ON, OFF, ON, OFF, OFF, OFF = 0110 1000 = RTU
address 104.
A unique RTU address must be assigned to each
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communicating on the same RS-485 port.
7.1.2 Modbus Data Registers and Function Codes
The following table identifies the
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Modbus register locations
and the function codes available:
Variable
Alias
Read Function Code
Write Function Code
FAULT BIT
12000
2
NA
0 = OK
1 = Fault
ALARMS
12008
2
NA
Returned as 8 discrete bits packed in the low byte of the response data.
12008:bit 0 = Fault (tracks 12000)
12008:bit 1 = Alarm 1
12008:bit 2 = Alarm 2
12008:bit 3 = Not used
12008:bit 4 = Not used
12008:bit 5 = Not used
12008:bit 6 = Alarm 2 acknowledge
12008:bit 7 = Not used
A2D Raw
33000
3 & 4
NA
10 bit value representing the A2D value of 0 to 1023 for -20 to 103% FS
(197=0% and 1003 = 100%).
