Isolated 4- to 20-mA transmitters (such as the IXR100 from Burr-Brown) allow you to build simple but sophisticated remote measuring systems. Often these systems have a built-in µP. One line’s limits in providing analog information about a measured process present difficulties if the remote system also needs to report its overall status. The simple watchdog circuit in Figure 1 allows you to use the same line to provide additional information about the system’s operation by closing or opening a 4- to 20-mA loop. A major advantage of this circuit is its full isolation from the host system. The isolation eliminates unwanted ground loops and other potentially damaging situations.

The watchdog circuit consists of a 74HCT123-retriggerable one shot and a relay with associated driving circuitry R₁, Q₁, and D₁. The rising edge of SELECT triggers the one shot to generate a pulse that closes the relay and the 4- to 20-mA loop. R₂ and C₂ determine the length of the pulse, which is about 2 sec. The remote system’s internal µP should select this circuitry at least 1 time/sec. Otherwise, after about 2 sec, the relay deenergizes and the current loop would open, thus reporting an erroneous condition to the host system. The circuit would also report if the system loses power or stops functioning. (DI #1813)