HSP-bugs.txt 7.3a        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. 
over.

        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        W1BEL@amsat.org
(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

