33
A
B
A'
B'
Rx
Tx
Master
Common
Earth
*
R*
R*
*
See
wiring
notes
Front
Remove link between terminals 90 and 91.
Slave(s)
(Boiler controller)
909192939495969798
H / DH / DRx+Rx-Tx+Tx-SCNCOM
R*
R*
Fig. 21 RS485 Modbus full duplex
circuit (view from the top)
EIA / TIA-485 wiring notes:
EIA / TIA-485 symbols are used (A = Tx-, B = Tx+ and A' = Rx-, B' = Rx+)
The signal direction is relative to the product being the Modbus slave, i.e. Tx+ from the product
(slave) is to be connected to the Rx+ of the master.
-
Twisted pair cable should not be required for short lengths of cable < 1.5 m (< 5 ft).
Standard screened cable should suffice.
-
The H/D (Half duplex) pins are used to select two or four-wire Modbus:
i) For two-wire mode, connect terminal 91 and 90 together.
ii) For four-wire mode, do not connect terminals 91 and 90 together.
Continued on page 34
5.7 EIA / TIA-485 communication wiring diagram
The product can be connected as a slave to a two or four-wire EIA / TIA-485 multi-drop network.
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