Microscan MS-890, User Manual

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MS-890 Industrial Automation Scanner User Manual
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Appendices
Appendix M — 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 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 (1219 m) 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.