RS-422/ 485 Connections/Operation
Manual Documentation Number 3PCIoUx-1008
Chapter 8
61
RS-422 Operation
In RS-422 mode, the transmitter is enabled (TX ENABLE) all the time, and
the receiver is enabled (RX ENABLE) all the time. Typical point-to-point
connections use a transmitter and receiver at each end with two wire pairs
connecting them. The transmit lines of the device at one end of the link are
connected to the matching receive lines of the device at the other end. The
second device transmit lines are connected to the receive lines of the first.
Another common connection mode—point to multipoints—connects the
transmit pair from the master device transmitter to the receive lines of many
listening slave devices. In this scenario the communications is one-way.
None of the listening devices have transmitters connected to reply to the
master.
RS-422 Limitations
The limitation of RS-422 operation is that only one transmitter can be
connected to a wire pair because the transmitter is active all the time. Even
when the transmitter is not sending data it will assume the idle, or Mark, state
in which TD(A)- is at 0V with respect to ground and TD(B) line is at about
4.4V with respect to ground. If another transmitter output is connected to the
same wire pair, and attempts to begin sending data by setting the line pair to
Space state, the first transmitter will hold the opposite state and neither can
communicate. To overcome this limitation, RS-485 mode was developed, in
which transmitters connected to the line are put into a high-impedance (tri-
state) state when not transmitting.
RS-485 Operation
RS-485 solves some of the limitations of RS-422, allowing multidrop
communications using 2-wire and 4-wire connections from multiple
transmitters and receivers.
To accomplish multidrop operation the transmit driver must be enabled only
during transmit (by asserting the enable input (TX SD) of the transmitter) and
tri-stated to a high impedance after the data has been sent. In the 2-wire (half
duplex) mode, the receiver is enabled when not transmitting, and disabled
(RX
) during transmit (called ‘echo off’ because it avoids having the
device receive its own transmissions).
