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3.3 RS-232 interface
An RS-232 interface is rated for distances up to 15 meters (50 feet). At least three wires
are required for an RS-232 interface. Wires are required for Transmit, Receive and Signal
Ground. Some devices support additional wires for communication handshaking. RS-232
hardware is a full-duplex configuration, having separate Transmit and Receive lines.
Each signal that transmits in an RS-232 data transmission system appears on the
interface connector as a voltage with reference to a signal ground. The RS-232 receiver
typically operates within the voltage range of +3 to +12 and -3 to -12 volts.
3.4 RS-422 interface
An RS-422 interface requires at least four wires. Two wires each are used for Transmit
and Receive. A fifth wire is usually required for Signal Ground, when connecting non-
isolated devices together. Handshaking lines may also be supported by some hardware.
This interface is full duplex, allowing use of the same software drivers as for RS-232. The
differential drivers allow for distances up to 1200 meters (4000 feet). The receivers of an
RS-422 device are always enabled.
3.5 RS-485 interface
For multi-drop operation, drivers must be capable of tri-state operation.
An RS-485 interface requires at least two wires. In a two-wire configuration, the same
pair of wires is used for Transmit and Receive. The two-wire configuration utilizes half-
duplex communications. Transmit driver circuits are always taken off-line or tri-stated,
when not in use. This tri-state feature reduces the load on the network, allowing more
devices, without the need of special hardware. This interface also uses differential
drivers, supporting distances up to 1200 meters (4000 feet).
In a differential system the voltage produced by the driver appears across a pair of signal
lines that transmit only one signal. A differential line driver will produce a voltage from 2
to 6 volts across its A and B output terminals and will have a signal ground (C)
connection. Although proper connection to the signal ground is important, it isn't used
by a differential line receiver in determining the logic state of the data line.
6 Serial interface considerations ç RS485
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