Fail-Safe Feature

A personal computer is a complex system, with hardware, peripherals, operating-system software, and application programs all working together.  When a computer is used to control a radio transmitter, there is the possibility that a failure of any one of these components could cause the transmitter to be keyed continuously.

EchoStation supports a unique fail-safe feature which helps guard against this possibility.  It uses a special, external circuit which allows the transmitter to be keyed only while a continuous "heartbeat" is being produced by the computer.  This heartbeat is generated from within the EchoStation software itself.  If the heartbeat stops, the transmitter is disabled.

There are two steps required for setting up the fail-safe feature:

  1. Build a simple circuit which sits between the computer and the transmitter.  An example of such a circuit is shown below. 
  2. Enable the fail-safe feature in the EchoStation configuration.  This is a check box on the Transmitter tab of the General Setup window.

With the fail-safe feature in place, the following types of failures should be protected:

  • Inadvertent keying of the transmitter before EchoStation starts.
  • Power surges and hardware failures which cause the computer to "lock up".
  • Operating-system, device-driver, and application-program crashes.

Technical Details

When the fail-safe feature is enabled, EchoStation sends a continuous train of pulses over the DTR line of the COM port, at a rate of 5 pulses per second.  These pulses are only sent while EchoStation is running and functioning normally.  The RTS line is used to key the transmitter as usual.  (EchoStation can be configured to swap DTR and RTS.)

The circuit shown below uses a retriggerable monostable multivibrator to accept these pulses from the DTR line.  The time constant of the one-shot is set to 5 seconds.  If no pulses have been received in the past 5 seconds, the transistor disables the RTS line so that the transmitter cannot be keyed.

A similar circuit can be built using a 555 timer chip, although a few more components are required since the 555 is not retriggerable.

In this example, the circuit connects to the computer's COM port and the existing PTT interface (such as a RIGblaster).  Of course, it is also possible to combine the fail-safe and PTT functions into a single circuit.