CHAPTER 2: Introduction
5
The RS-232
↔
RS-485 Interface Converter
provides bi-directional synchronous or
asynchronous conversion for all commonly used
RS-232 and RS-485 signals. The unit is designed
with one port configured as Data Terminal
Equipment (DTE) and the other as Data
Communications Equipment (DCE). Operation is
not recommended with both ports configured
either DCE or DTE when operating in
synchronous mode.
The unit has two jumper-selectable configurations:
one for connecting RS-485 modem equipment to
RS-232 terminal equipment (DTE to DCE), and
one for connecting RS-232 modem equipment to
RS-485 terminal equipment (DCE to DTE). Both
configurations allow bi-directional data transfer.
A two-wire half-duplex application is shown in
Figure 2-1, followed by a four-wire full-duplex
application diagram in Figure 2-2. An RS-232
cable configuration for an IBM
®
PC is shown in
Figure 2-3.
2.0 Introduction
IBM PC
OR OTHER
DTE DEVICE
RS-232/RS-485
INTERFACE
CONVERTER
RS-232 PORT = DCE
RS-485 PORT = DTE
PINS 4 & 6
JUMPERED
ON IC
RS-485
DEVICE
DCE
+B
-A
PINS 22 & 24
ARE JUMPERED
ON THE INTERFACE
CONVERTER
DB25
STRAIGHT
CABLE
Figure 2-1. 2-Wire Asynchronous Application
NOTE
The two- and four-wire application
diagrams illustrate only the wiring of
the data leads. All other leads are still
passed through from one port to the
other.
NOTE
Echo is not disabled in 2-wire half-
duplex operation. A carrier signal or
transmitter being enabled on the
RS-485 line will not raise the RTS or CD
signals on the RS-232 port.
NOTE
In a two-wire application, the toggling
of the RTS lead from the RS-232 port
enables and disables the RS-485
transmitter. Because hardware
handshaking (RTS) is required by the
RS-232 standard in order to control the
RS-485 drivers, and RTS is not
supported by most communications
packages despite the fact that it is
software-dependent, you may need a
customized software package in order
to implement any 2-wire application.
