APRS HSP Port Switch

If You Have Difficulty Getting the APRS Port Switch to Work There are two common problems to look for: The unit is not getting power or the GPS signals are at too low a level.

Use a voltmeter to determine if there is DC Voltage coming in from either the PC connector pin 6 or the TNC connector pin 6. If not, check for another pin on the PC or TNC which can provide 6-12 VDC and change the cable wiring accordingly.

If power is not available from the PC or TNC, then a 9 Volt battery may be connected to supply power between Pin 6 (+) and Pin 5 (-) of the RS-232 (PC) connector. This is also a good way to get the unit operating initially after trouble is experienced.

The other most likely cause of problems is inadequate drive in the positive direction from the GPS receiver. In some cases, DTR is always a little too high and this is the next most likely cause of trouble. The circuit works only on positive going signals. The GPS data output is fed through a 1k resistor and must have adequate drive capability to get at least 0.7V on the base of transistor TR3 in order to turn it on. This is normally the case, but SOME GPS receivers don't seem to do this. Adding a 4k7 resistor between the data input and pin 6 of either the TNC or computer port of the adapter should provide enough pull-up to make it work. The series 1k resistor on the GPS input cannot be lowered in value because TR4 is used to hard clamp the data and would short circuit the GPS output.....

DTR is used to switch the circuit between data sources. This needs to go at least 1.2V POSITIVE when the circuit is in the TNC mode. On some computers the low state was still too high and would cause TR4 to remain turned on all the time, thereby preventing any GPS data to be gathered. A simplistic fix to this was to add a second diode in line with DD5 and thereby drop 0.7V. A better fix would be to vary the value of RD9 up a little.

Troubleshooting is performed by using APRS in HSP mode. Select Ops- Comm-TNC to see TNC data (O-C-T). Use the F8 key to toggle the HSP switch. The OPS-COMM-GPS command is for dual-port TNC's but will also toggle the HSP switch to the GPS data.

Revision History for PacComm APRS Port Switch Units

ECN 97 11/30/94: Add a 1N914 diode is series with D7 to work better with GPS inputs that have minimal negative voltage swing.

ECN 103 3/13/95: Connect Pin 6 of the TNC connector to Pin 6 of the PC connector to provide an additional source of supply voltage for computers with no wiring to Pin 6. PacComm TNCs provide +5 VDC on pin 6.

HECN 104 3/16/95: Change RD8 from 1k to 4.7k to improve TR2 switching reliability.

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Gwyn Reedy, W1BEL gwyn@paccomm.com
(813) 874-2980 Fax:+(813) 872-8696 BBS: +(813) 874-3078 (V.34)
PacComm Packet Radio Systems, Inc, 4413 N. Hesperides St, Tampa, FL 33614-7618

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