Microscan MS-5000, User Manual, Page: 86 / 94

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MS-5000 Multidrop Concentrator User’s Manual
Appe
ndices
Appendix F — Interface Standards
Interface Standards, established by the Electronic Industries Association
(EIA), specify such things as the signaling voltage levels, maximum cable
lengths, and number of drivers. With Microscan devices, selection of inter-
face is made by pin assignment and, in the case of the host communica-
tions, by software switching between RS-232 and RS-422. Microscan
devices use RS-232, RS-422, and RS-485 multidrop.
RS-232
RS-232 defines an interface between two devices such as, for example, the
concentrator and host. It differs from the other interfaces by dedicating indi-
vidual pins to specific functions and by requiring both devices to share a
common ground line. Since both device chassis are connected to a com-
mon ground, a ground loop potential and the possibility of noise interfer-
ence exists. Therefore cable lengths are limited to a maximum of 50 feet
(15.24 m). Despite being the most limited, this interface is used frequently
because of the large installed base of RS-232 equipment.
RS-422
RS-422, unlike RS-232, measures signals differentially that is, the receiver
looks at the potentials between the two receive (or transmit) wires rather
than the potential between signal and ground. As a result, cables, if
shielded, can be up to 4000 feet (1219 m) in length. Like RS-232, RS-422
communication is designed for only two devices on a single line. It can be
used wherever RS-232 is used.
RS-485
RS-485, like RS-422, can transmit up to 4000 feet (1219 m) using differen-
tial voltages but unlike RS-422, its transmitters are turned off until a request
for data is received from the host. RS-485 is used exclusively in Multidrop
protocol.
Note: “Communications Menu” in chapter 3, “Menu Configuration,” has addi-
tional information on standards.