B&B Electronics 3PCIOU4 - Manual - Manual (Page 39)

RS-422/ 485 Connections/Operation
Document Number 3PCI-0712m Chapter 8 33
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 TDA(-) is at 0V with respect
to ground and TDB(+) 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).
Figure 16. An RS-485 Two-Wire Multidrop Connection
Send Data Control
MIport cards provide Send Data Control (SDC) for the RS-485 driver and receiver. This
is hardware controlled based on the contents of the UART output buffer. When data is
present, the driver is enabled; when the output buffer becomes empty, it is disabled.
This automatically handles whatever baud rate is used.
The RS-485 transmitter and receiver have separate settings for 2-wire modes (TX SD,
RX SD) or 4-wire (TX SD, RX ENABLE).