2.4
Data Communications with the Watlow Series 988 Family
Hardware and Wiring, Chapter 2
Wiring
RS-422 Interface Wiring
The RS-422 communications uses a five-wire, full-duplex system. There are two
separate lines for transmitting, two lines for receiving and a common line between
the computer and the controller. With RS-422 you can connect from one to ten
controllers to a single computer.
This diagram is a typical wiring example for units shipped after 1993 (see
ç
Caution on this page). The connections to the converter box or computer may
vary, depending on the model. Refer to the documentation for specific information.
NOTE:
The Electronic
Industry Association
(EIA) RS-422
standard recom-
mends a maximum
network distance of
4,000 feet.
Figure 2.4 -
RS-422 Interface
Wiring Diagrams.
Com
T-
T+
R-
R+
B'
A'
B
A
Gnd
Converter Box or Card
Controller #1
Com 7
R-
6
R+
5
T-
4
T+
3
T+
3
T-
4
R+
5
R-
6
Com 7
As many as 10 units
can be connected to
an RS-422 network.
Controller #2
If the system does not work properly it may need
termination resistors across the receive A and B termi-
nals at the converter. A typical value would be 240
Ω
.
Pull-up and pull-down resistors may be needed to
maintain the correct voltage during the idle state.
+5V
Converter
box with
termination
pull-up and
pull-down
resistors.
B
A
B
A
RD
TD
1K
Ω
Gnd
1K
Ω
240
Ω
ç
CAUTION:
For older Series
986-989 control-
lers with a "date
code" of 4693 or
earlier, terminal
3 is T-, 4 is T+, 5
is R- and 6 is
R+. (See Diag-
nostics Menu in
the User's
Manual.)
T-
3
T+
4
R-
5
R+
6
Com 7
Controller
