Altair-Duino 680 A6 LED issues

[Christopher McNabb]

I'm seeing some odd issues with the A6 LED on a freshly built Altair-Duino 680.

Board revision 1.0.3

 Here are the symptoms:

At power  on with HLT switch in HLT:  All address LEDs are off except A6

Raise reset, All LEDs go off.

Raise A6 - A6 LED stays off

Raise any other address switch:   corresponding LED and A6 come on.

Lower any raised address switch:  corresponding LED goes off, A6 stays on.

I've looked for solder bridges and unsoldered connections but didn't find any.  

Has anyone seen a this or a similar issue?

[Christopher McNabb]

One additional symptom:

With switches A4,A5,A6,A7 down and holding reset down while connecting USB, the baud rate comes up at 4800.  I'm leaning towards the LED issue being more of a switch/input issue. 

[Chris Davis]

That's a really useful clue, and I think you're right — this is now looking much more like an A6 switch/input problem than an LED problem.

When RESET is held during power-up, switches A4-A7 select the baud rate. With all four switches down, the value should be 0000, which selects 9600 baud.

4800 baud corresponds to 0100 — in other words, the firmware thinks A6 is raised.

That also fits with the A6 LED being on when the machine is halted. It may simply be displaying what the Arduino believes the switch setting is.

A6 is read on Arduino Due pin D68. The switch inputs use the Due's internal pullups, so with A6 down/off I would expect D68 to measure roughly 3.3V, and with A6 raised/on it should be pulled to approximately 0V.

Could you measure D68 in both switch positions?

If it stays near 0V in both positions, I'd look closely at the A6 switch, its solder joints/traces, and the connection to D68 for a short to ground.

Also measure continuity on the A6 switch.  When the switch is down, the top two pins should be connected, when it's up the bottom two pins should be connected.

[Christopher McNabb]

Measuring across the A6 switch  in-circuit I had 0 ohms between the top, middle, and bottom lugs all at the same time regardless of switch position.  After I desoldered and removed the switch the A6 light stayed out and the system behaved as it should. 

Measuring the switch out of the circuit sometimes it would act like it should and sometimes it showed continuity between all lugs regardless of position.

So, I guess I need to get a new switch.

  

[Chris Davis]

Take that switch and flip it back and forth several dozen times.  When they sit in storage they can develop oxidation that can interfere with the operation.  Measure for continuity after that.

[Christopher McNabb]

I think this one is definitely bad.  I can make it open and close just by touching the side without moving the toggle

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[Chris Davis]

Ok I'll send one out today.

[Christopher McNabb]

The replacement switch arrived yesterday and was quickly installed.  Everything works now.  Thanks for the quick response!

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