No joy with home made cable - twice - any ideas? Resolved

[gtg]

I have now made tt's JP1.2 cable twice, first was a bit scrappy on veroboard, so I gave up debugging that and went for an identical build on perf board.

Nothing works software wise and although I have done extensive testing, I cannot find a problem. Tester and com_test also fail.

To confirm, the cable checks out as it should, 1k from 1-4 and 1-2, oc everwhere else at the remote end. 10k 2-4, diode 2-5, 3-5 diode + 100k etc.

The remote blinks twice when the interface is plugged in, and again during tests, but everything reports no JP1.x interface found. The remote continues to function normally with the interface connected and after it is removed. URC-7950, and URC-7555 give the same results.

On the HP laptop DTR is 6.5v high and -5.6v low so I'm not sure if the interface would pull down pin 2 low enough.

Using a LED I can see DTR, TX data but no RX data at the DB9 end.

I can see RX data at the remote by connecting the LED across 1&4 - the LED is on but flickers when I send something, but I can't get anything anywhere on the return leg.

The transistors I'm using are 2n3904's so I'm assuming emitter-base-collector.

Laptop comport checks out OK with a loopack test.

Could it be the HP laptop is just one of those that aren't compatable?





Some pics to clarify...

[gtg]

Well, judging by the lack of immediate response I can assume there's nothing obviously wrong with the cable. So having proved the transmit leg, I had some thoughts about testing the receive leg.

The problem in testing was the lack of a reliable data stream from the remote when testing, so rather than use the loopback/battery method as suggested by TT, I decided to disconnect 4 & 6 from the remote and join them together to form a loopback at the remote end.

As I can see transmit data using a LED across R7 I should be able to see receive data using the same method? Sure, a LED across R6 is on, pulsing off when data is received, suggesting Q4 is switching suggesting that the serial port is ignoring data.

Adding a 1k resistor across R6 to increase the current and a LED allows the com_test to work and the JP1xTEST to pass.

IR - however continues to report no response from interface.

Any suggestions welcome....

[gtg]

Getting a bit lonely in here...

Having scratched my head (a lot) and fiddled around with resistors diodes and leds with varying success I came to the conclusion that there was not enought grunt from the DTR line to trigger RX data on my laptop, perhaps due to current limiting at the serial interface. Although by varying componant values I could get JPXTEST and com_test to pass, IR continued to fail.

My best guess was that by reducing the value of R6, RX data could be made to work, but DTR gets dragged down enough to trigger a reset.

Plan "B", or maybe plan "W" it's been a long day, was to cut R6 where it joins DTR and feed a separate power rail for Q4 from a 9v battery

Voila - success - back to the drawing board before we go in to mass production TT.

[greenough1]

Great work on your issue. I wonder if issues other have with certain laptops are due to a similar problem?

Best,
jeff