Microscan MS-4X Instrukcja Użytkownika Pobierz pdf (Strona 292)

Appendix O

A-36 MS-880 Industrial Long Range Scanner User’s Manual

Appendices

Appendix O — Interface Standards

Interface Standards, established by the Electronic Industries Association (EIA), spec-

ify 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 maxi-

mum of 50 feet (19.7m). 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

(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 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.

DeviceNet

DeviceNet is a versatile, general purpose FieldBus designed to satisfy 80% of the

most common machine- and cell-level wiring requirements. One of DeviceNet's major

benefits is its multiple messaging formats, which allow the bus to 'work smart' instead

of work hard.

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