Appendices
A-18
MS-850 Raster Scanner User’s Manual
A
p
p
e
ndice
s
Appendix I — 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
functions 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 max-
imum of 50 feet (19.7 m). Despite being the most limited, this interface is used fre-
quently 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
(1219 m) 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 (1219 m) using differential volt-
ages 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.