Documentation Number 485TBLED3301 - pg. 1/2
B&B Electronics -- Revised September 2001
This product designed and manufactured in USA of domestic and imported parts by
RS-232 to RS-485 Converter
CE
Model 485TBLED
The 485TBLED converts unbalanced, full-duplex RS-232 signals to 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 or four-wire system.
The RS-232 port, configured as a DCE port, has a female DB25 connector with pins 2 (TD input), 3 (RD output), 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. Pins
1 (Frame Ground) and 7 (Signal Ground) are connected straight through to the RS-485 terminal blocks. The RS-485 terminal blocks
support Transmit Data (A-) and (B+), Receive Data (A-) and (B+), Frame Ground, Signal Ground, and +12 VDC input. See
Schematic.
LEDs
The 485TBLED has two LEDs: a Transmit Data LED to show when the RS-485 driver is enabled and a Receive Data LED
showing data appearing at pin 3 of the RS-232 port. These are very useful for determining if data is getting through the converter.
Note that the TD LED indicates that the RS-485 driver is enabled. Data must be present on pin 2 of the RS-232 side for data to be
transmitted out of the RS-485 side. If no data is present, no data will be transmitted even though the TD LED is illuminated.
Flow Control
The 485TBLED uses two different methods to enable the
RS-485 driver, either by toggling Request to Send (pin 4) of
the RS-232 side, or by automatic sensing of the data on
Transmit Data (pin 2) of the RS-232 side. This option is user
selectable by setting push-on jumpers located next to the
terminal blocks. See Fig. 1. Removing both sets of jumpers
completely can also constantly enable the RS-485 driver and
receiver. This makes the 485TBLED act like an RS-422
converter.
Baud Rate
There is a timing component on the converter, a resistor
R9, see Fig. 1. This component is part of the automatic
sensing circuit, and affects the baud rate at which the
converter can be used in a two-wire setup. This component, is
factory selected to allow the converter to run at 9600 baud or
higher. With this component, the RS-485 driver will shut off
approximately 1ms after the last character has been sent. If you need to change the 485TBLED to a baud rate other than 9600
baud, you can change this component. To change the baud rate, remove
R9 and add through hole component R14, see Table 1.
Figure 2 shows how to interconnect two RS-485 converters using 4-Wire and 2- Wires.
Figure 2
Figure 1
Documentation Number 485TBLED3301 - pg. 2/2
B&B Electronics -- Revised September 2001
This product designed and manufactured in USA of domestic and imported parts by
The Echo jumper 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. 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.
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 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
24AWG twisted pair telephone cable with a shunt capacitance of 16 picofarads per foot, and no more than 4000 feet of distance.
Specifications:
Dimensions
2.14" x 3.32" x .725" (54.5 x 84.34 x 18.43mm)
Supply Voltage 9 14 Vdc
Temp Range 0 C 70 C
Data rates up to 115.2 kbps
Connector DB25 Female on RS-232 side
Table 1
Resistor Replacement For
Changing Baud Rate Timeouts
Baud
Rate
Time
(ms)
Resistor
R9/R14
(ohm)
1200
8.33
820K
2400
4.16
430K
4800
2.08
200K
9600
1.04
100K
19200
.520
56K
38400
.260
27K
57600
.176
16K
115200
.0868
8.2K
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:
485TBLED
Description:
RS-232 to RS-485 Converter
Type:
Light industrial ITE equipment
Application of Council Directive: 89/336/EEC
Standards:
EN 50082-1:1998
EN 50082-1:1992 IEC 801 (-2, -3, -4)
EN 61000 4 (-2, -3, -4, -6)
Michael J. Fahrion, Director of Engineering
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