|
Q: I want to use a pair of 485LDRC, 485LDRC9 or
485DRC Converters to Isolate and Extend RS-232. How do I interconnect
them?
A: It depends
on the two devices being connected, whether they are (1) DTE
or DCE devices, (2) how they are connected together before
adding RS-232/RS-485 Converters, and (3) how many signal lines
are required by the devices. You can use these converters
if your RS-232 device is “3-wire” and requires
only Receive (Rx), Transmit (Tx) and Ground connections. Some
devices will require connections for RTS/CTS and DTR/DSR,
so 4 channel or 8 channel RS-422 converters must be used instead.
A2:
See example figure below: Set switches 1-4 OFF for RS422 mode.
- The RS-232 device on the left side has DTE pinouts which
match a computer with DB9M connector. Pin 3 is output (Tx),
pin 2 is input (Rx), pin 5 is ground. Connect as Shown.
If you have the 485LDRC9, you can connect directly to a
computer or DTE device with a straight through cable, the
DB9 connections include loopback.. See table for DB25.
- The RS-232 device on the right side has DCE pinouts similar
to a modem, so pin 3 on the device is input (Tx), pin 2
is output (Rx), 5 is ground, note crossover. If using the
485LDRC9, you will need to use the terminal connections
or wire a crossover cable to the DB9.
How to identify which of two lines is the Output
from the RS-232 device:
Take a DC voltmeter, measure from the ground wire to each
of the other two RS-232 wires, while the RS-232 device is
powered up. Usually one lead has a minus (-) DC voltage, typically
between –11 volts and –3 volts. Whichever lead
has a minus voltage is the lead to connect to our Terminal
D. The other lead usually has nothing or noise relative to
ground. If neither lead has a minus (or positive) voltage
on it relative to ground, recheck for OPEN cable connections
to the RS-232 device or the device pinouts. If the device
can be configured multiple ways, make sure all the jumpers
and such are set to RS-232.
A few devices may use very low power RS-232 ports which switch
only between Ground and positive, so to identify which is
line is active, the device must be set to transmit, then connect
the ground wire to the 485LDRC and one of the wires to D,
see if the TD indicator flashes. If not, try the other wire.
If the data request is coming from the other end, and all
that wiring is correct, and if it is being polled, the RD
indicator should be flashing. Some DC meters may show a slight
flicker of DC or AC voltage on a data line with changing data.
Set switches 1 to 4 OFF. This sets the receiver always On,
and Transmitter to RS-422 mode which is proper for one device
at each end. There is no need to tri-state the transmitter
because the line pair does not need to be shared with another
device. In this mode, the baud rate switches have no effect,
leave them at default.
Full Duplex 4-wire operation works best for RS-232 signals.
Half Duplex operation using “2-wire” connections
is usually NOT satisfactory because the RS-485 bus takes time
to “turn-around” between transmit and receive,
so some of the first bytes sent may be lost. RS-232 devices
don’t wait before responding to a data request, but
RS-485 devices designed for 2-wire operation wait.
This information should help you make the needed connections.
Customers typically use this connection for Fire Alarm panels
serial port extension, and other
RS-232 devices for weight or signaling between locations.
|
