Document No. 485COSR10812 - pg. 1/2
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PRODUCT INFORMATION B&B ELECTRONICS
RS-232 to RS-485 Converter
Model 485COSR1
The RS-232 to RS-485 converter converts unbalanced, full-duplex RS-232
signals to balanced full-duplex (4-wire), or half-duplex (2-wire) RS-485 signals. RS-
485 is an enhanced version of the RS-422 balanced line standard.
The RS-232 port has a female DB25S type of connector with pins 2 (TD
input) and 3(RD output) supported. Protective Ground (pin 1) and Signal Ground
(pin 7) are also passed through. Pins 4(RTS) and 5(CTS) are tied together. Also
pins 6(DSR), 8(CD), and 20(DTR) are tied together, but not passed through the converter. The RS-485 port has a male
DB25P type of connector with the Send Data outputs on pins 2 and 14, and Receive Data inputs on pins 3 and 16 or 5
and 17.
Figure 2 shows how to interconnect two RS-485 Converters using two signal
wires. The resistors Rt are optional, depending on the line length, baud rate, etc.
The resistors should be about the impedance of the line used, but in no case
should they be less than 120 ohms each. No special software requirements are
needed, since the 485 driver is enabled by the first transition on the RS-232 side
of the transmit data line (Pin 2). Any transition on the TD line keeps the RS-485
driver enabled by preventing the monostable multivibrator from timing out. The
transmitter is disabled approximately 1 ms after the last transmitted character.
This 1 ms timeout allows continuous transmission of data at 9600 baud or higher.
If other baud rates are required, see Table 1.
The 485COSR1 is configured so that thle receiver is always
enabled with a 2-wire system. Data transmitted from the RS-232 port will
be echoed back to the RS-232 port. A jumper wire from pin 18 to pin 21
can be installed to disable the receiver during transmission. Up to 32
receivers can be driven by any one driver. This allows you to put together
large systems with many drop points.The termination resistors should be located approximately at opposite ends of the
system.
Proper operation of any RS-485 system requires the presence of a signal return path. The RS-485 Standard
recommends that a third wire be used for this. For safety, a 100 ohm half-watt resistor should be connected
between pin 7 and the "reference" wire at every drop point. While it may be possible to interconnect signal grounds
(pin 7's) directly, this is not recommended due to the danger of circulating currents possibly being present.
Table 1
COMPONENT REPLACEMENTS FOR
CHANGING BAUD RATE TIMEOUTS
Baud
Rate
Time
(ms)
Resistor
(R6)
(ohm)
Capacitor
(C7)
(mfd)
300 33.3 330K 0.1
600 16.6 160K 0.1
1200 8.33 820K 0.01
2400 4.16 430K 0.01
4800 2.08 200K 0.01
9600 1.04 100K 0.01
19200 .520 56K 0.01
38400 .260 27K 0.01
57600 .176 16K 0.01
115200 .0868 8.2K 0.01
(8 pages)
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