SATEC 1771DB - Manual (Page 6)

6
For RS-422 balanced data transmission, a twisted pair cable should be used for each
communication link. To minimize reflections and reduce cross talk, it is recommended to
terminate the ends of lines with the termination resistor of 120
Ω
, as shown in Appendix B.2.1.
When lines are routed through electrically noisy environment, input protection against
switching or lightning induced surge voltages is required in addition to line termination. Surge
voltages can be generated by switching operations in power substations, or can occur as a
result of voltage deep fade, phase gating controls, contactor relay controls, etc. Transient
surge voltage effects may put the BASIC module or PLC out of action and in the worst case
lead to destruction of unprotected electronic components. A diagram of the input protective
circuit is shown in Appendix B.2.2. You can also use complete surge voltage protection units
for RS-422 communication available from Phoenix or from other manufacturers of similar
protective equipment.