Technical information

IM-P405-39 AB Issue 4
21
Rx
Tx
Master
Common
Earth*
R*
R*
* See wiring notes
Front
Slave(s)
(Boiler controller)
9091929394959697E
H / DH / DRx+Rx-Tx+Tx-SCNCOM
Fig. 9 RS485 / Modbus half duplex circuit (view from the top)
Add link between
terminals 91 and 90
EIA / TIA-485 wiring notes continued:
- The bus common must be connected directly to protective ground / earth at one point only.
Generally this point is at or near the master device.
- Consider terminating the two furthest ends of the bus to match the transmission line
impedance. A 150 ohm (0.5 W) resistor or a 120 ohm (0.25 W) resistor, which is decoupled
with a 1 nF (10 V) capacitor is commonly used, but ideally the line impedance should be
matched to each individual installation. Termination for short lengths of cable should not
be necessary < 300 m (< 1 000 ft) @ 9 600 Baud.
- See 'Technical information' for cable details.