Appendices
MS-860 Industrial Raster Reader User’s Manual
A-25
Appendix K — 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 RS232 and
RS422. Microscan devices use RS232, RS422, and RS485 multidrop.
RS232
RS232 defines an interface between two devices, such as the reader 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 maximum of 50 feet
(19.7m). Despite being the most limited, this interface is used frequently because of
the large installed base of RS232 equipment.
RS422
RS422, unlike RS232, 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 RS232, RS422 communication is designed for only two devices
on a single line and must have a common ground. It can be used wherever RS232 is
used.
RS485
RS485, like RS422, can transmit up to 4000 feet (1219 m) using differential voltages,
but unlike RS422, its transmitters are turned off until a request for data is received
from the host. RS485 is used exclusively in multidrop protocol.
USB
The Universal Serial Bus can connect up to 127 devices to a host at a maximum of 6
megabits per second (Mbps) of bandwidth.
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