IM-P337- 6 9 EMM Issue 2
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5.3. EIA/TIA-485 communication wiring
dc power wiring
Connect a 24 Vdc supply (+/-10%, 250 mA min) to the +24V and 0V terminals on the terminal block.
The supply must be fitted with a T1A fuse.
WARNING:
Ensure the 4-20mA loop is not wired up, if connecting power to the DC power input.
Signal wiring
The product can be connected as a slave to a two or four-wire EIA/TIA-485 multi-drop network.
Note: 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.
Cable specification:
Type: Screened EIA RS485 Twisted pair
Number of pairs: 2 or 3
Gauge: 0.23mm
2
(24 AWG)
Maximum length: 1 200m (4 000ft)
Recommended type: Alpha Wire 6413 or 6414
Note: Note: LAN Category 5 or Category 5E ScTP (screened), FTP (foil) or STP (shielded) cable can be
used, but limited to 600 m.
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.
Termination resistors should not be necessary for 1200 Baud.
For two wire communications ensure the terminals Rx+ to Tx+ are linked and terminal Rx- to Tx- are liked.
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