Document No. 485OISPR4300 - pg. 1/2
!
1993 B&B Electronics - Revised November 2000
This product designed and manufactured in USA of domestic and imported parts by
Reversed RS-232 to RS-485 Optically Isolated Converter
CE
With Surge Suppression
Model 485OISPR
The Model 485OISPR converts unbalanced, full or half-duplex RS-232 signals to optically
isolated, balanced, full or half-duplex RS-422 or RS-485 signals. RS-485 is an enhanced version of
the RS-422 Standard. It allows multiple drivers and receivers on a two-wire system.
The 485OISPR is an RS-232 to RS-422 converter with optical isolation and surge protection. The
opto-isolators are rated up to 2500 VAC. The surge suppressors are 7.5 V, bi-directional avalanche
breakdown devices, with a 500W peak power dissipation and 1 picosecond (theoretical) clamping time. The surge suppressors add a
maximum of 6000 picofarads.
The RS-232 port has a female DB-25 connector with pins 2 (TD), 3 (RD), and 7 (Signal Ground) supported. Pins 4 (RTS) and 20 (DTR)
are used to power the RS-232 side of the converter. If your port cannot supply this power, a power supply can be connected to the RS-232
pins 25 (+12 VDC) and 12 (ground). The RS-485 DB-25 male connector provides connection for Transmit Data (A) and (B), Receive Data (A)
and (B), and Signal Ground. There is a power supply, which plugs into the side of the converter, available from B&B for the RS-485 side of the
converter. On the 485 side, power may also be supplied to pins 25 (+12 VDC) and 12 (Ground).
The 485OISPR enables the RS-485 driver by automatic sensing of data on Transmit Data (pin 2) of the RS-232 side. The 485OISPR has
two jumpers located inside of the box. One enables either RS-422 or RS-485 mode (JP4). The other (JP2) is used in the two-wire mode, and
allows you to prevent data being sent from the RS-232 port from being echoed back to the RS-232 port. See Figure 2.
There are also two components internal to the converter, a resistor (R15) and a capacitor (C9), that control timing. These components are
part of the automatic sensing circuit, and affect the baud rate at which the converter can be used in a two-wire setup. These components are
factory selected to allow the converter to run at 9600 baud or higher. With these two components, the RS-485 driver will shut off
approximately 1ms after the last character has been sent. If you need a baud rate lower than 9600 baud or would like to configure the
485OISPR for a specific baud rate, see Table 1.
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:
485OISPR
Description:
Reversed Opt. Isolated RS-485 Converter
w/Surge Suppressors
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 1
COMPONENT REPLACEMENTS FOR
CHANGING BAUD RATE TIMEOUTS
Baud Rate
Time
(ms)
Resistor
(R15)
(ohm)
Capacitor
(C9)
(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
Document No. 485OISPR4300 - pg. 2/2
!
1993 B&B Electronics - Revised November 2000
This product designed and manufactured in USA of domestic and imported parts by
Up to 32 receivers can
be driven by any one RS-485
driver, allowing you to put
together large systems with
many drop points. If you are
using termination resistors,
they should be located at
opposite ends of the system.
Figure 3 shows how to
interconnect two RS-485
converters in a half-duplex
system using two wires. The
resistors Rt are optional,
depending on line length,
baud rate, etc. The resistors
Rt should be about the
impedance of the line used,
which is normally about 120
ohms each.
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. For safety, a
100 ohm, 1/2 watt resistor
should be connected between
Signal Ground and the
"reference wire" at every drop
point. While it may be
possible to interconnect
Signal Grounds 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 picofarads
per foot, and no more than
4000 feet long.
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