Campbell Scientific SC932 9-Pin to RS-232-DCE Interface - Manual (Page 14)

SC932 9-Pin to RS-232-DCE Interface
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transient protection shown may be purchased from Campbell Scientific,
Inc. (p.n. 5563 shown in Figure 7, p.n. 6536 includes a plastic case, p.n.
6361 includes hardware for mounting to ground lug of CSI enclosures
models ENC 10/12, ENC 12/14, or ENC 16/18). Spark gap wiring is
straight through such that pin to pin continuity exists between the two
modems. If the modems are installed entirely within a building, the
transient spark gap protection is probably not needed.
FIGURE 7. Installation of Spark Gap Protection