Microscan MS-810, User Manual

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MS-810 Industrial Scanner User’s Manual
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Appendix L — 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 interface is made by pin assignment 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 multidrop.
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 func-
tions and by requiring both devices to share a common ground line. 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 (19.7m). 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 deferentially; 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
(1219m) in length. Like RS-232, RS-422 communication is designed for only two
devices on a single line and must have a common ground. It can be used wherever
RS-232 is used.
RS-485
RS-485, like RS-422, can transmit up to 4000 feet (1219m) using differential 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.