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Selecting an EIA/TIA-232 to EIA/TIA-485 Converter
When choosing an EIA/TIA 232 to 485 converter, look for
one with the following features:
Two-wire capability
EIA/TIA-485 can be implemented as a two-wire
system or a four-wire system. Most Watlow control-
lers, including the Series SD, use two-wire com-
munications when working with EIA/TIA-485. The
converter selected must have a two-wire mode.
Some converters can only be used in a four-wire
mode.
Automatic Send Data control
In a two-wire system, both the transmitted signals
and the received signals travel over the same pair
of wires, so the converter must have a method of
changing from the transmit mode to the receive
mode. Some converters require the toggling of a
control line (usually the RTS line) to perform this
transition, while others use an automatic timing
circuit. The toggling method is dependent on the
PC software to toggle the control line and the PC's
operating system to make that transition happen
in a timely manner. Because of these dependencies,
the best choice for a converter is one with automat-
ic control.
Isolation
Converters are available with or without input-to-
output isolation. An isolated converter is not a re-
quirement when used with the Series SD, but it is
recommended to avoid ground loops. Isolation could
be a consideration when the Series SD will be used
on a network with other devices that may require
isolation.
Power Supply
Many converters can be powered up either through
the signals of a serial port or through an external
power supply. Because some computers, such as
laptops, do not always provide enough power to
supply the converter, we recommend using an ex-
ternal power supply with specifications as recom-
mended by the converter manufacturer. Isolated
converters may require two supplies.
Biasing and termination
If the system does not work properly, it may need
termination resistors at each end of the network.
A typical installation would require a 120-ohm
resistor across the transmit/receive terminals (3
and 4) of the last controller in the network and the
converter box. Pull-up and pull-down resistors may
be needed at the converter to maintain the correct
voltage during the idle state. The pull-up resistor
is connected between the positive of the DC supply
and the T+/R+ terminal. The pull-down resistor is
connected between the negative of the DC supply
and the T-/R- terminal.
Wat lo w Se rie s S D
Wat lo w Se rie s S D
485SD9TB
Watlow p/n 0830-0473-0001
6 ft. comms cable -
Watlow p/n 0830-0473-0003
Figure 18a — B&B Converters
Isolated Converter - 4850I9TB
Non-Isolated Converter - 485SD9TB
B&B Electronics Manufacturing Company,
(815) 433-5100, http://www.bb-elec.cm/
Figure 18b — CMC Non-Isolated Converter - ADA485L
CMC Connecticut Micro-Computer, Inc.,
1-800-426-2872, http://www.2cmc.com/
NOTE:
The CMC converter requires an external power supply when
used with a laptop computer.
Power Supply
DC
-VÎ
+VÎ
-VÎ
+VÎ
B
EIA/TIA 485
Converter
A
GND
Figure 18c — Wiring bias and termination resistors.
Controllers must be wired in a daisy chain configuration.
Add a 120Ω termination resistor on the last controller.
1 8 •
18
•
•
•
T-/R-
T+/R+
Power
–
Supply
+
Watlow p/n 0830-0473-0002
120V
9V
(dc) (see note)
G
9V
COM.
G
T+/R+
B
T-/R-
A
B
A
7-ft. comms cable -
Watlow p/n 0219-0217-0000
SD
SD
3
3
4
4
1KΩ
T+/R+
120Ω
T-/R-
1KΩ
Chapter 2 Install and W i re
Tabl e of Conte nts
4
3
120V
(ac)
(ac)
3
4
SD
3
4
120Ω
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