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Model 8322 - RS-232 to RS-485 (4-wire) Interface Converter DIN Rail Mounted Reference Manual

Table of Contents



The Model 8322 RS-232 to RS-485 interface converter allows information exchange in half-duplex mode over two twisted pair cable. It converts RS-485 signals to RS-232 signals, bidirectionally, on the DIN rail mounting system.
The RS-485 standard specified by the Electronic Industries Association (EIA) relates to the electrical characteristics of the generators and receivers used in a multipoint system with symmetrical or balanced digital signals. The RS-485 interface allows the user to control up to 32 devices in a half duplex via a four wire, two twisted pair line.

The Model 8322 has one RS-232 I/O configured in DCE mode enabling the reception of data circulating on the receive pair (RS-485 side) or the transmission of data towards the RS-485 transmit pair. The principle of the RS-485 4-wire connection involves a half-duplex transmission mode, i.e. at a given time only one device is authorized to transmit data. Because this also utilizes a polling procedure it is also referred to as a master-slave network. The other stations (slaves) must be in stand-by mode, which corresponds to a 'high impedance' state for each driver. The control of the driver is generally realized from the RTS signal, although the Model 8322 also allows control by sampling Transmit Data. This feature is described below in Section 2. The control of the devices in RS-485 mode requires multipoint software with polling sequences. The secondary stations controlled by the master station cannot transmit data unless they are selected.
IMPORTANT : The 4-wire RS-485 link which connects the various equipment requires a cable with at least 2 twisted pairs, nominal impedance of 120 Ohms and general shielding. The use of a cable with different characteristics can lead to unreliable operation, especially in case of high data rates and long distance.


2-1 Block Diagram:

The Model 8322 interface converter provides opto isolation between the RS-232 interface and the RS-485 line by the use of optical couplers. The transmit data from the RS-232 port can be sent over the RS-485 line only if the RS-485 driver is in active mode. This active mode can be initialized either by activation of the RTS signal from the RS-232 port or by detection of transitions on the transmit data lead (Pin 2 TD). This feature enables the Model 8322 adapter to support the XON/XOFF flow control from equipment using only the TD, RD and GND.

2-2 RS-485 4-Wire Connection

The principle of 4-wire connection (2 twisted pairs) enables the management of up to 32 peripheral devices. The principle of an RS-485 4-wire link is based upon the use of independent transmission and reception circuits. Each link requires a terminating load on the ends of the cable. Each Model 8322 includes a switch selectable termination. Those units which are located at the ends of the cable should have this 120 Ohms termination (RS-485 Standard) selected.
The intermediate secondary stations can use a balancing termination by connecting a 'pull-up' resistor to the negative line side of the bus and a 'pull-down' resistor to the positive side of the bus, thereby forcing a zero condition on the line. The secondary station transmission circuits present the characteristic of operating in 3 state mode, a high impedance state during the idle period (period of non data transfer).
The sending of data from one station can be initiated by the incoming RTS signal (Pin 4) on the RS-232 side going active, or by internal generation of a data received signal on the RS-232 side. This second method, controlled by a microprocessor, enables the use of the Model 8322 converter with a simple 3-wire RS-232 connection and XON/XOFF flow control.

3-2 RS-485 HDX operation mode :

The lack of a control circuit on the RS232 side requires that the Model 8322 take control of managing the driver for transmission of data towards the RS-485 line. Detecting a start bit sets off the driver immediately (t0), remaining active until a half-bit following the last stop bit. If a new start bit is detected during 't2' time, the driver is kept active until the next character and so on. The internal microprocessor controls the t0 and t2 times in accordance with the transmission parameters indicated by the user via the S1 DIP switch (see § 4-3).


3-1 RS-232 I/O (DCE mode)

3-2 4-Wire RS-485 I/O

3-3 DIN rail mounting of Model 8322

Top View :

Front View :



The Model 8322 incorporates a micro-controller to manage the host protocols of the interface. Many of the parameters controlling the micro are established via dip switches. Some of these settings are marked mandatory and should not be changed. Other switches that are user selectable are appropriately marked by an X.


01 : Mandatory
01 : Mandatory
010 :
001 :
  • :
Transmission control with RTS
Transmission control without RTS
Other combinations are not allowed
0 :
1 :
Common not connected to Earth by 100 Ohms
Common connected to Earth by 100 Ohms


0 :
1 :
Load Mid-point not connected to Signal Ground
Load Mid-point connected to Signal Ground
01 : HDX mode, mandatory
0 :
1 :
No PULL-UP on Receive -
PULL-UP on Receive - in multipoint
0 :
1 :
No PULL-DOWN on Receive +
PULL-DOWN on Receive + in multipoint
00 :
11 :
Terminal Loads not connected
Terminal Loads connected
0 :
1 :

Common not connected to Loads
Load Mid-point towards common

4-3 DIP-switch S3

Generally the use of an RS-485 interface implies controlling the Half-duplex operation mode. The DIP-switch block S1 enables the selection of the operating mode and data transmission characteristics needed by the microprocessor in detecting and managing the active phases of the RS-485 transceiver (see Section 2-3).

Transmitter Configuration S3-1 to S3-3 Baud Rate, S3-4 to S3-5 Byte Format

Speed S3-1 S3-2 S3-3
300bps 0 0 0
600bps 1 0 0
1200bps 0 1 0
2400bps 1 1 0
4800bps 0 0 1
9600bps 1 0 1
19200bps 0 1 1
38400bps 1 1 1

Format S3-4 S3-5
7 bits, 1 stop, no parity 0 0
7 bits, 1 stop, with parity 1 0
7 bits, 2 stops, no parity 1 0
7 bits, 2 stop, with parity 0 1
8 bits, 1 stop, no parity 1 0
8 bits, 1 stop, with parity 0 1
8 bits, 2 stop, no parity 0 1
8 bits, 2 stop, with parity 1 1

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