24
AC7A/B User’s Guide
Figure 3-12: Group B and C RS-422/485 Jumper Schematic
More details on these jumpers is included in later sections of this manual.
CONNECTING THE AC7A/B TO AN RS-422/485 DEVICE
For this connection, Opto 22 recommends the use of three pairs of shielded, twisted wire (a total of six wires).
All conductors should be stranded #22 or #24 gauge, with a nominal impedance of 100 ohms and a capacitance of
12.5 pF/ft.
The first pair of wires is used for the transmit line. The second pair of wires is used for the receive line. The third pair
of wires is used to connect the LOGIC GND of the AC7A/B to the COM terminal of the RS-485 device. Finally, the
shield of the three pairs of twisted wire should be tied to earth ground at only one end of the shield.
Figure 3-13: RS-422/485 Cabling Diagram
A slightly less noise-immune connection may be made with a two twisted-pair shielded cable. Instead, use the
shield drain wire of the cable to connect logic ground/common between the two devices. In this case,
DO NOT
connect the shield to earth ground.
INSTALLATION (cont.)
