10
DUAL PORT RS-232↔422/485 SERIAL INTERFACE
Figure 4-3. DB9 Handshake Loopback connections for RS-232
(shown from rear or cable side of connector).
4.3 RS-422/485 Interface Options
To select the RS-422/485 mode of operation, install DIP shunts in the sockets
found at E6 and E7. E6 sets Port 1 and E7 sets Port 2.
Figure 4-4 shows Headers E3 and E4. Position “A” determines whether the
RS-485 driver is enabled by the UART signal Request To Send (RTS) or always
enabled. With the jumper installed in position “A,” RTS enables the driver.
Removing the jumper enables the driver, regardless of RTS. E4 sets Port 1
and E3 sets Port 2. This jumper should be set to “A” only if you are running
the board in a multidrop polled environment such as RS-485, and you have
software that “knows how to talk” on the RS-485 bus. For normal point-to-
point RS-422 (such as terminal emulation), make sure that a jumper at
position “A” is not in place.
Positions “B” and “C” determine whether the board provides a direct ground
connection (as in RS-232 and most RS-422), or a 100-ohm high-impedance
ground. The high-impedance ground is normally used by
RS-485 (and some RS-422) to avoid ground-loop currents with long cables.
Position “B” selects the direct ground, and position “C” selects the 100-ohm
high-impedance ground. “C” should be selected when RS-422/485 operation
is selected.
