B&B Electronics 485COSR Datasheet

Document No. 485COSR0812 - pg. 1/2

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International Office: 707 Dayton Road PO Box 1040 Ottawa, IL 61350 USA 815-433-5100 Fax [phone]

PRODUCT INFORMATION B&B ELECTRONICS

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

DECLARATION OF CONFORMITY

Manufacturer’s Name: B&B Electronics Manufacturing Company
Manufacturer’s Address: P.O. Box 1040

707 Dayton Road
Ottawa, IL 61350 USA

Model Number: 485COSR
Description: 2-Channel RS-485 Converter w/SD Control
Type: Light industrial ITE equipment
Application of Council Directive: 89/336/EEC
Standards: EN 50082-1 (IEC 801-2, IEC 801-3, IEC 801-4)

EN 50081-1 (EN 55022, IEC 1000-4-2)
EN 61000 (-4-2, -4-3, -4-4, -4-5, -4-6, -4-8, -4-11)
ENV 50204
EN 55024

Robert M. Paratore, Director of Engineering

RS-232 to RS-485 Converter

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 cas e
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 485COSR has an internal connection to prevent data transmitted from the
RS-232 port from being echoed back to the RS-232 port. The connection can be
removed to have the receiver always enabled. (Refer to schematic) After the
connection is removed, 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.
No wire type or maximum run length is listed in the RS-485 Standard.
However, the RS-422 Standard, which is very similar, recommends number 24 AWG
twisted-pair telephone cable with a shunt capacitance of 16 picofarad per foot and no
more than 4000 ft. of distance.
The polarity of the two RS-485 lines must be correct. With no data being sent,
the RS-232 line should be negative and the RS-485 "A" terminal should be negative
with respect to the "B" terminal. If your equipment uses a "+" and "-" naming
scheme, in most cases the "A" line will be connected to the "-" and the "B" line will be
connected to the “+”.

Document No. 485COSR0812 - pg. 2/2

www.bb-elec.com [e-mail] [e-mail]

International Office: 707 Dayton Road PO Box 1040 Ottawa, IL 61350 USA 815-433-5100 Fax [phone]

PRODUCT INFORMATION B&B ELECTRONICS