The APRS Telemetry Interface
The Three Watchdogs
The interface has three watchdogs to monitor the correct operation of the
interface itself and the PC.
- Internal Watchdog:
The controller has an internal watchdog. It
has an oszillator and a counter, working independent of all other hardware
inside the controller. If a counter overflow occures, after about 2s, the
wachdog will restart the controller and the software will work in normal state
again. This case should never happen as the software resets the counter in
regular times, shorter than 2s. If the software crashes, the counter will not
be reset and the watchdog will restart the software.
If the program DIGI_NED is working correctly, it will
send packets in regular times. Even if no one uses the digipeater, it will
send its beacon every 30 minutes or what you have configured. You can check
the correct operation of DIGI_NED measuring the time since its last
transmission. If this time is longer than 35 minutes, the software has crashed
and you have to reset the PC to restart the program for restoring normal
operation. The interface does this measurement for you. You can set up the
If DIGI_NED crashes just while transmitting, the
PTT-line will not be reset. The digipeater will transmit for a longer time and
this will cause a lot of trouble on the frequency. Length of normal
APRS-packets is about a second. If transmision is longer than 10s, the
software on the PC has crashed and the system has to be restarted. The
interface does this measurement too. You can set up the time.
The reset signal for the PC is generated by a pulse of 1-2 seconds on
If you have activated the watchdog function, DIGI_NED must be running on
the PC, your modem and radio must work correctly. If not, there will be no
correct PTT-signal and the watchdog will reset the PC after the watchdog time.
You will not want this if your digipeater is not working. You can set the
watchdog mode to 0 or cut off the reset line between the interface and the PC.
Connecting to the PTT-line
If you want to use the watchdog function, you have ot connect the input
"WD-In" to the PTT-line of your modem. There are two choices
- Connect the input "WD-In" to the PTT-line inside your modem:
The modem has a small transistor to switch the PTT-line of the
transmitter. This transistor has a resistor at its base. Connect the input of
this resistor to the input "WD-In" of the interface. If the modem is
transmitting, there should be a positive voltage of some volts. On receiving
there should be about 0V.
- Connect the modem to the microphone jack of the radio:
input "WD-In" to the PTT-line of the radio, parallel to the modem.
While transmitting there will be about 0V. In case of receiving there should
be a positive valtage of some volts. The voltage at the microphone jack must
not be negative.
Voltage and current vary in a wide range on different radios. May be the
connection to the microphone jack does not work. In this case try the other
- Mode 0:
The PC-watchdog is off. The input "WD-In" can be used for event
counter 3 or as a simple digital input.
- Mode 1:
The watchdog is working and it is controlled by the signal from inside
the modem. Receiving time is checked while the input "WD-In" is 0V.
Transmitting time is checked while positive voltage is applied to the input.
Event counter 3 counts the packets, sent since startup of the interface. Low
Frequency Counter 3 and Extremely Low Frequency Counter 3 count the number of
transmissions in their gate times.
- Mode 2:
The watchdog is working and it is controlled by the signal from the
microphone jack on the radio. Receiving time is checked while positive voltage
is applied to the input "WD-In". Transmitting time is checked while
0V at the input. Event counter 3 counts the packets, sent since startup of the
interface. Low Frequency Counter 3 and Extremely Low Frequency Counter 3 count
the number of transmissions in their gate times.
Reset cirquit for the PC
A little relay has to be connected to transistor N2 (WD-Out). Its contact has
to be connected to the reset-pins of the PC, parallel to the reset-button.
Depending on the type of relay you will need 5V or 12V for supply. Get this
voltage directly from the input or output of the voltage regulator 7805. Do
not forget the supressor diode.