Thank you all for your rapid responses.
I tried the GPS2Clie.html link
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and it had valuable information. I tried things out right away, but still had no luck. Here are the results:
- on opening the case of the HotSync cable, I noticed surface mount components- the inside of the cable looked exactly the same as in the picture in section 3 of the URL
- on the DB9 I connected pin 5 to ground, and 6 volts to pin 4, as instructed, and later tried pins 7 and 8 both individually and together with pin 4, with the same results, namely, that the output on the TX pin (3) changed between 0 and 2.1 volts when sending data. This was enough, when I connected pins 2 and 3 together, to enable the PDA to send data to itself- the data was sent and received by the PDA itself.
- when I tried to plug the PDA into the breadboarded project via the MAX233, the voltage levels were of course not RS232 level, and there was no response from the PIC. When I cut out the middleman MAX233 and fed the signals directly into the PIC serial inputs, I got an erratic response. The approximately 2.1 volts of the PDA output was apparently too "gray" to get reliable communications.
I tried boosting the voltage on pin 4 to 9v, and got no better response either through the MAX233 or directly to the PIC.
This leaves me with several questions.
- Why are the level shifters (I assume that is what the SMD components on the HotSync cable are) not working when I power them up through pin 4? Are they supposed to transform the PDA voltages into RS232? They certainly seem to when connected to a PC!
- I connected my PC serial cable to the breadboarded project and measured the voltage comming out of pin 4 without anything connected to pin 4. The voltage was -11v (!). I am sure that the polarity was correct on my meter. -11v was coming out of the PC on pin 4. What gives? I am supposed to put +6v on this pin to feed power to the level shifter circuits in the cable. I am missing something basic but I don't know what.
That is where it stands. I look forward to your responses. Thank you.