Document No. 485OIC4300 - pg. 1/2

� B&B Electronics 1993 -- Revised November 2000

This product designed and manufactured in USA of domestic and imported parts by

Optically Isolated RS-232 to RS-485 Converter

Model 485OIC

The Model 485OIC converts unbalanced, full or half-duplex RS-232 signals to optically

isolated, balanced, full or half-duplex RS-422 or RS-485 signals at baud rates up to 38400 bps.
RS-485 is an enhanced version of the RS-422 Standard. It allows multiple drivers and receivers on
a two-wire system.

The RS-232 port has a male DB25 connector with pins 2(TD), 3(RD), and 7(Signal Ground) supported. Pins 4(RTS) and 5(CTS)

are tied together, and pins 6(DSR), 8(CD), and 20(DTR) are also tied together. These handshake lines provide power for the RS-232
side of the isolator. If you have a port that cannot supply these pins, or if they cannot provide enough power, a power supply can be
connected to the DB-25 connector on pins 25(+) and 12(-) to power the RS-232 side of the converter. Terminal blocks are provided for
Transmit Data (A) and (B), Receive Data (A) and (B), RS-485 power and ground as shown in Figure 1. Note that the RS-485 Ground
terminal block is the connection for RS-485 Signal Ground as well as the RS-485 power ground. Figure 2 shows how to connect the
converter to a two-wire or four-wire system. The resistors Rt (labeled R6 on the converter) are termination resistors that should only be
used when very long cable runs are used at high baud rates. If installed, the resistors should equal the characteristic impedance of the
line used, normally about 120 ohms. Refer to B&B's RS-422/485 Application Note for more information on termination and biasing.

The 485OIC enables the RS-485 driver by automatic sensing of the data on Transmit

Data (pin 2) of the RS-232 side. The 485OIC has two jumpers located adjacent to the
terminal blocks. JP2 enables either RS-422 or RS-485 mode. Placing JP2 in the shorting
position enables RS-485 mode, removing the jumper switches to RS-422 mode. JP1 is
used in two wire connections, preventing data being sent from the RS-232 port from being
echoed back to the RS-232 port. Placing JP1 in the shorting position will prevent the data
from echoing. Removing the jumper will hold the receiver enabled. Refer to Table 2 for
typical jumper settings.

There are two internal components, resistor R12 and capacitor C7, which control

timing of the automatic send data circuit. These components are factory selected to allow
the converter to run at 9600 baud or higher, disabling the RS-485 driver 1 ms after the last
character has been sent. To change to a baud rate lower than 9600 baud, or to configure
the 485OIC for a specific baud rate, these component values need to be changed. Due to
the surface mount construction of the converter, space for through-hole components has
been provided on the opposite side of the PCBD as R12 and C7. These through-hole
components are electrically in parallel with R12 and C7. To change to a baud rate other
than 9600, remove R12 and C7, select the proper values for the baud rate from Table 1
and install them in the spaces labeled R18 and C14.

Up to 32 receivers can be driven by any one RS-485 driver, allowing you to put together

large systems with many drop points. 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
24AWG twisted pair telephone cable with a shunt capacitance of 16 picofarads per foot, and
no more than 4000 feet long. Proper operation of any RS-485 system requires the presence
of a return path. The RS-485 Standard recommends that a third wire be used for this.

Figure 1: Timing Component Locations

Table 1. Timing Component Values

Baud
Rate

Time
(ms)

R18
(

)

C14
(

�

300

33.0

330K

0.1

600

16.0

160K

0.1

1200

8.2

820K

0.01

2400

4.3

430K

0.01

4800

2.1

200K

0.01

9600

1.04

100K

0.01

19200

0.6

56K

0.01

38400

0.3

27K

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 Numbers:

485OIC

Description:

RS-232/485 Optically Isolated Converter

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

Michael J. Fahrion, Director of Engineering

Table 2. Jumper Settings

JP1

JP2

RS-422

Off

RS-485 2-wire

RS-485 4-wire

Off

Электронный компонент: MODEL485OIC

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