Wednesday, February 08, 2012

shared the good news

As Phil and I enjoyed the HOV lane down to the Ontario Science Centre, we reflected on our Losmandy cable adventure. He was pretty happy to hear I had finally met with success. And was looking forward to getting his computer to work with it (even though he didn't foresee using it a lot).

Still we puzzled over what we had done wrong. Despite four eyes and two brains on it. When something floated to the surface. Something I had not thought about from a long time... One other variable in the equation of COM port serial communications: DTE.

Maybe the GPS receiver was a DTE device! Phil frowned.

I explained to Phil that when connecting a computer serial port to an old modem, one would use a "standard" straight-through cable. For it was designed to interconnect the PC (considered Data Terminal Equipment, if I remember my terms correctly) to a traditional peripheral device, say a modem or a printer (considered Data Communication Equipment). You'd select a DTE-DCE cable. But if you were trying to connect two computers directly, say when using the old LapLink app (Phil nodded in simpatico), you'd had a DTE-DTE cable, aka a null-modem. And that essentially the only thing different in these two cables was the transmit and receive pin outs.

The more I thought about it, the more it made sense.

We had used his GPS cable for our pin out analysis when we should have treated it suspect. We should have gone straight to the Gemini manual. I should have remembered while considering the other serial port settings, like speed and handshaking, that DTE or DCE interfacing might be a factor. And maybe, if I had been thinking ahead, I would have considered all these issues in advance and raided my old computer cables parts bin for serial cables, adapters, gender benders, and handy-dandy null-modem adapters!

Too much on my brain this week...

Frustrating on one hand. But pretty satisfying.

Thank you John Bramwell for introducing me to modems 30 years ago!

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