Reznor, Guardian GB GR and GS, Installation Manual, EN 2021-08, D301214 Iss D Page No 33
C
MODBUS Protocol
The modbus hardware is configured as follows:
• RS485 serial half-duplex interface
• 9600 baud
• 8 bits
• even parity / no parity (see below)
• 1 start bit
• 1 stop bit
• RTU mode
This means the same software will run in a standard
panel driven display and also in modbus mode, but
in a modbus installation only 8 maximum controllers
may be supported.
Modbus address
– the unit will respond to it’s own
coded address as set by the bitswitches as follows: It
will also respond to ’broadcast’ mode 0.
Even parity settings
(Bitswitch3 OFF)
No parity settings
(Bitswitch3 ON)
Modbus
Address
Switch
Setting
Modbus
Address
Switch
Setting
16
0
24
8
17
1
25
9
18
2
26
A
19
3
27
B
20
4
28
C
21
5
29
D
22
6
30
E
23
7
31
F
Modbus function codes supported
Codes
Descriptions
Range of
arguments
01, 02
Read coils, read discrete
inputs (equivalent)
Coils 1 to 24
03, 04
Read Holding registers,
input registers
(equivalent)
Registers 121 to
126
05
Write single coil
Coils 13 to 24
06
Write single register
Registers 125, 126
All other MODBUS function codes will generate
exception code 01, function not recognised.
Function arguments — Registers
0
Controller type
Read only
(undefined)
122
System control
temperature
Read only
123
Outside air
temperature
Read only
124
Power ratio
Read only
125
Set point
temperature
Read/write
126
Set point
temperature on
door down
Read/write
(always to
nearest 5°C)
127
Hours run
Read only
(16 bit result)
