Appendices
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MS-610 Scanner User’s Manual
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Appendix H — 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 (see table A-3 on page A-3) and, in the
case of the host communications, by software switching between RS-232
and RS-422. Microscan devices use RS-232, RS-422, and RS-485 multi-
drop.
RS-232
RS-232 defines an interface between two devices such as, for example, the
scanner and host. It differs from the other interfaces by dedicating individual
pins to specific functions and by requiring both devices to share a common
ground line (pin 4). Since both device chassis are connected to a common
ground, a ground loop potential and the possibility of noise interference
exists. Therefore cable lengths are limited to a maximum of 50 feet
(19.7 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 (1575 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 (1575 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:
See “Protocol,” on page 2-6 for additional information on standards.
