35
Rx
Tx
Master
Common
Earth*
R*
R*
* See wiring notes
Front
Slave(s)
(Boiler controller)
909192939495969798
H/ DH/ DRx+Rx-Tx+Tx-SCNCOM
Fig. 27 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, in series 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 Section 10 - 'Technical information' for cable details.
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