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ISA Bus Isolated RS-422/485 Communications and Watchdog Card


  • Watchdog timer to monitor computer systems
  • Opto-Isolated RS-422/485 Serial Communications port
  • Supports conventional High-speed RS-422, and Multi-drop RS-485
  • Can be wired for 2 or 4 wire RS-422/485 communications
  • Supports RS-485 handshaking on-board without special drivers to support Windows and non-DOS programs
  • Built-in support for local echo, line termination, and line bias supplies
  • Watchdog temperature alarm
  • LED status monitors on transmit and receive lines for troubleshooting
  • Designed, made, supported, and manufactured in the USA

This multifunction card contains a Watchdog Timer, an RS422/485 Serial Communications port, and an internal Temperature Alarm. The WDG-SIO is a full-length card that installs in "long" expansion slots of IBM PC/XT/AT and compatible computers. The following paragraphs describe functions provided by the WDG-SIO card.


It's a fact of life that computers can fail. If a computer fails it can cause catastrophic damage. There are two methods to reduce risk of computer failure; (a) redundancy and (b) a watchdog circuit. Neither method offers 100% assurance but both of these methods reduce risk or consequences of failure. Redundancy, a duplication of computer circuitry, is very expensive. On the other hand, ACCES' Watchdog card offers excellent protection from temporary malfunctions at very low cost.

An application program must communicate with the watchdog circuit at prescribed intervals. If this communication ("prompt") is missed, the Watchdog will initiate a computer reset. This restarts the computer from the beginning of the program. If the failure was temporary, proper operation is resumed. If, however, the failure is persistent, the Watchdog will continuously reset the computer. The more frequently the computer is prompted (and shorter Watchdog time selected), the less time a faulty computer has to cause damage.

The method used by the WDG-SIO card to determine loss of computer function is as follows:

  1. A counter/timer has a number greater than zero set into it by your application program.
  2. This counter/timer counts down toward zero at a 225 Hz rate.
  3. As long as the computer is functioning properly, the number in the counter/timer is periodically re-entered by your application program before the counter/timer reaches zero.
  4. If this software resetting of the timer/counter fails to occur, the timer/counter reaches zero and a hardware reset of the computer is attempted.

Implementation of this watchdog procedure can be accomplished by your application program, by AUTOEXEC.BAT, or by other appropriate software.

A type 8254 counter/timer chip is used in the Watchdog circuit. The clock source for this chip is 225 HZ derived from a crystal oscillator on the card and is independent of the computer clock. The watchdog time-out is software programmable from 5 mSec to 291 seconds.

The address for the Watchdog circuit is completely independent of the Serial Communication address. It is jumper selectable anywhere within the I/O address range 000 to 3FF hex.

The output of the Watchdog circuit is an open-collector transistor output rated at up to 100 mA. If Option S01is included, then a Form C relay output is provided at a terminal board. Terminal 1 connects to the normally-closed contact which is returned to ground through the relay center arm. Terminal 3 is connected to the normally-open contact until the relay is activated by a time-out at which time it is connected to ground by the relay center arm.


The 8254 counter/timer chip contains three 16-bit counters. Counter/timer 2 is used by the Watchdog circuit. If your card has Option S03 is included, then there is access to counter/timers 0 and 1 via a 9-pin I/O connector at the rear of the computer. On-board clock frequencies of 1.843 MHz, 230 KHz, 28.8 KHz, 3.6 KHz, and 225 Hz are available by DIP switch selection. Counter/timer gate inputs and counter/timer outputs are via pins on the I/O connector.

If Option S03 is included, outputs of counter/timers 0 and 1 are buffered by drivers capable of 120 mA drive and are available at pins 1 and 6 respectively of the 9-pin connector. If Option S04 is included, those outputs are also opto-isolated. If Option S05 was specified, then relays are installed instead of opto- isolators. Relay K3 actuates on counter/timer 0 and K2 operates on counter/timer 1 outputs. When a relay actuates, the normally-open contacts at I/O connector pins 5 and 4 respectively are closed to pin 2 of the 9-pin I/O connector.


The WDG-SIO card also contains a temperature alarm circuit that activates if the internal temperature in the computer becomes excessive. That alarm can initiate an interrupt so that an early shutdown may be accomplished.

The alarm is factory set at 50oC. You can adjust this if you desire.


The Serial Interface function of the WDG-SIO card can be used for either RS422 serial communications or RS485 communications. (The RS485 specification allows multiple transmitters and receivers to communicate over a two-wire "party line" bus.) Opto-isolators are incorporated on this card to provide common-mode voltage isolation on the Tx, Rx, RTS, and CTS communications lines.

Type 16550 UART's are used as the Asynchronous Communication Element (ACE). Use of the same ACE as used in IBM original equipment makes the card 100% compatible with existing programs when the base address is set as either COM-1 or COM-2. However, use of the Serial Interface is not restricted to COM-1 or COM-2 only. Different addresses can be selected anywhere within the I/O address range 100-3FF hex.

An on-board crystal oscillator permits precise selection of baud rate from 50 to 9600.

The output transceiver used, the new generation type 75176, is capable of driving extremely long communication lines at high baud rates. It can drive 60 mA on balanced lines and can receive input signals as low as 200 mV amplitude superimposed on common mode noise of maximum -7V/+12V. In case of communication conflict, the transceivers feature thermal shutdown.

The communication lines are loaded at the receiver and biased at the transmitter. Also, an on-board DC-DC converter provides isolated power to the transceiver and opto-isolators are provided in the serial I/O lines.

Two LED indicators are provided in the Serial Interface circuit. These LED's blink to indicate activity on the transmitting and receiving lines and are useful for problem diagnosis.

In addition to dual, differential Transmit and Receive lines, single-ended, buffered RTS and CTS lines are provided on the I/O connector. The RTS line can be used to control the Transmitter and Receiver. The CTS line can be used to check for proper installation of the communication cable. To check for proper cable connection, introduce +5VDC to the CTS line on the cable side of the connector. Then read the CTS bit by software. Signal ground and +5 VDC are available at the I/O connector.

Full duplex, half duplex, or simplex configuration can be selected by jumper options.

Available Reference Manuals

Available Software Downloads

Drivers and Downloads

Full list of available Downloads: Software Packages, Drivers, Manuals, and other documents

Custom Software

ACCES also offers Custom Software Services for our products. Our prices are unbelievably low, often as inexpensive as free! If you need something tweaked to support your needs, or an entire enterprise application developed from scratch, it is definitely worth your time to inquire with us, first.

Further information about available ACCES Software:

Redistributing Windows Drivers
A list of ACCES drivers and the files that compose them under different versions of Windows, so you can easily redistribute ACCES cards and drivers.


Watchdog Timer

  • Time-out: Software selectable from 5 mSec to 291 Sec in 5 mSec increments.
  • Output Pulse Width: 4 msec.
  • Clock: 225 Hz, crystal controlled.
  • Address: Continuously mappable within 000 to 3FF hex I/O range.
  • Relay Option: Contacts rated at 250 mA at up to 24VDC.

Temperature Alarm

  • Temperature: Factory preset at 50 oC Adjustable.
  • Interrupts: Jumper selectable from IRQ2 through IRQ7, IRQ10 through IRQ12, IRQ14 or IRQ15
  • Alternate Output: Can be read on data bit 0

Counter Output Option
(Counters 0 and1)

  • Standard Output: Open collector transistor, 100 mA
  • Opto-Isolated Output Option: 2 mA at up to 24VDC
  • Relay Output Option: Contacts rated at 250 mA at up to 24VDC

Communications Interface

  • I/O Connection: 25-pin male D-sub connector compatible with RS422 and RS485 specifications
  • Input Isolation: 60 Volts, signal to ground and signal to signal.
  • Character length: 5, 6, 7, or 8 bits.
  • Parity: Even, odd or none.
  • Stop Interval: 1, 1.5, or 2 bits.
  • Serial Data Rates: 50 to 9600 baud (Faster rates available), Asynchronous, Type 16550 buffered UART.
  • Address: Continuously mappable within 000 to 3FF (hex) range of AT I/O bus addresses.

RS422/RS485 Differential Communication Mode

  • Multipoint: Compatible with RS422 and RS485 specifications. Up to 32 drivers and receivers allowed on line. Serial communications ACE used is type 16550.
  • Receiver Input Sensitivity: +200 mV, differential input.
  • Common Mode Rejection: +12V to -7V
  • Transmitter Output Drive Capability: 60 mA, with thermal shutdown.


  • Operating Temperature: 0 to +60 degree C.
  • Storage Temperature: -50 to +120 degree C.
  • Humidity: 10% to 90% non condensing.
  • Size: 13.3 inches. (330mm)

Power Requirements

  • +5 VDC at 420 mA, 500 mA maximum

Regulatory Compliance

  • This product is in full compliance with CE requirements.

ModelPrice (USD)
Option S0115.00
Option S0210.00
Option S0315.00
Option S0420.00
Option S0525.00

Acquisition, Control, Communication: Engineering and Systems