B&B Electronics PCWDT PC Watchdog Timer Card

B&B's Watchdog Timer cards are designed to overcome the dangers or annoyances associated with PC's locking up.



PC's used in remote or dial-up applications, used for controlling external devices or performing data acquisition can all benefit from a watchdog. If the controlling software hangs or crashes, the watchdog will timeout and reboot the host machine (just like pressing the reset button). The application software must reset the card's counters every timeout period to prevent the watchdog from rebooting the computer - the reset commands are identical for each of the watchdog models. The timeout period is selected by writing four bits to a register, selectable from 0.5 seconds to over 500 seconds, and can be further extended with a minor modification. The read registers allow you to check the current status of the card. Each watchdog card has a SPDT Form C relay output with a rating of 115 VAC @ 0.5A, allowing an external alarm or other device to be controlled.

Model PCWDT - B&B's original Watchdog Timer can be configured with any address from 0 to 3F8h, with no IRQ needed. This model uses a DB25 connector for external connections and includes a non-splicing connection wire to make the connection from the PCWDT to the motherboard's reset pins. A 4-bit write only register is used to set the timeout period. An isolated input is provided for resetting the card's counters with an external signal instead of using a software trigger. Works under DOS, Windows 9x and Windows NT. Sample code is included.

The PCWDT is periodically reset by your software or an external hardware trigger. If the watchdog doesn't receive the reset trigger within a software selectable timeout period, the PCWDT will "push" the reset button of the computer.

A form C relay output is also provided to reset an external device if desired.


• Software selectable timeout period from 0.5 to 550 seconds.

Range can be increased with a simple hardware modification.

• Software enable and disable.

• Form C relay output to control external device.

• Non-splicing connection to motherboard reset pins.

• Isolated timer reset input, for use as alternate trigger source.


Dimensions: 2.6" H x 4.1" L

Address: Occupies 1 byte of address space, selectable from 0 to 3F8h

Power: 250 mW maximum

Input specs: Optically isolated input, requires 5 - 12 volts.

50 μs minimum pulse width

Output specs: Form C relay, SPDT output, maximum ratings

24 VDC at 1.0 A

115 VAC at 0.5 A

Connector: DB25 male


In order for the PCWDT to reboot the host computer, a connection must be made in parallel with the reset switch to the reset pins on the motherboard. A jumper wire is provided with the PCWDT to make this connection. Note that some PCs do not include these pins on the motherboard. An additional input and output connection can be made through the DB25 connector. A form C relay output is provided for connection to an external alarm or to shut down another device. An isolated input is also provided to allow an external device to control the PCWDT.


The PCWDT works as a countdown timer that can be programmed to start its countdown from 0.5 seconds up to 550 seconds. A simple command, such as an OUT statement in Basic, must be added to a loop in the host computers software. Every time this statement is executed, the timer is reset to its maximum value and restarts its countdown (an external hardware trigger can also perform this function). If the code quits executing for any reason, the counter will countdown until zero is reached, rebooting the host computer and energizing the output relay.
    • UPC 3662458001991
    • Brand B&B
    • Mfg Part # PCWDT
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