Author Topic: ABBA switch, the other way  (Read 5914 times)

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Offline gerald

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ABBA switch, the other way
« on: 19:37, 06 August 12 »
Hi all

Since some SW do not work from B drive, and I am not keen on integrating a 3 1/2 drive into my 6128, I finally decided (15year later :o ) to do an ABBA switch.
However, the usual solution involve a bit too much cable and track cutting for my taste.

Looking at the 6128 schematics, I looks like there is an other way.

Drive selection is a bit non standard as DS1 is inverted DS0.
To swap internal and external drive, you just need to have a controlable inverter between the FDC and the DS0 open collector driver.
That is controlable inverter is available on board as a free XOR gate in IC209 (pin 4, 5, 6). IC209 is part of ROM7 address decoder.

What is needed :
  2 resistor 2.2k, 10k (I've used SMD) but regular 1/4W is OK
  1 switch
  few wires

How to proceed :
  1. cut track between FDC (IC201.29). Best place is close to IC209 pin 29
  2. connect IC201.29 to IC209.5
  3. connect IC209.6   to the via on track from IC209.29 to IC206.9
  4. connect the resistances from IC209.4 /  IC209.6 to 5V
  5. connect the switch between IC209.6 and GND

When the switch is closed, the internal drive is A
When the switch is open, the internal drive is B


end notes :
  1. This should apply to DDI1 as well (schematic is identical). EDIT : Gate is used by 4MHz to 8MHz clock generation for FDC data separator
  2. This does not apply to 6128+ as the XOR gate is now merged in ASIC
  3. This may not apply on cost down CPC for same resason as 2. I do not have any to check
« Last Edit: 14:54, 02 September 12 by gerald »

Offline IanS

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Re: ABBA switch, the other way
« Reply #1 on: 20:12, 06 August 12 »
Seems like a really nice solution, have you actually wired it up yet?

I'm slightly suprised the inputs to the unsed gate aren't tied to gnd on the pcb, floating inputs generally not a good idea.

Offline gerald

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Re: ABBA switch, the other way
« Reply #2 on: 20:28, 06 August 12 »
It's up and running, but I closed the CPC before taking any picture  >:(
Floating input is not a big issue on TTL gate.

That spare gate seems to have been forgotten by the designer as it does not appear anywhere on the schematic.
There is also a spare inverter (IC117) but this one has its input tied to ground because it a CMOS gate.

Offline gerald

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Re: ABBA switch, the other way
« Reply #3 on: 21:08, 06 August 12 »
Pictures !!!

Bottom side

* PCB_Top.JPG
(142.12 kB, 1007x524 - viewed 773 times)

Sharp eyes will notice that I did not cut the track as described  :P But where I did the cut is more risky.

Top side
 [ Invalid Attachment ]
Switch is on bottom left near the audio jack connector. Wires goes under the board and are connected to lyre socket pins on IC209. This allow removing the PCB from without messing with cable.

Offline IanS

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Re: ABBA switch, the other way
« Reply #4 on: 21:12, 06 August 12 »
I'd put off adding a switch for same reasons, but I'll have to give this a try.

(What is the other board with the crystal on over the 8255?)

Offline gerald

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Re: ABBA switch, the other way
« Reply #5 on: 21:23, 06 August 12 »
(What is the other board with the crystal on over the 8255?)

That's a PS2 keyboard adapter.

Offline Bryce

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Re: ABBA switch, the other way
« Reply #6 on: 22:28, 06 August 12 »
Why are you linking pin 4 to the output via the 10K Resistor? Shouldn't this be tied to the 5V rail instead? And is the 2.2K really necessary? You could probably link it directly. Nice solution though, I think I'll use it on my 6128 too :)

Bryce.

Offline gerald

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Re: ABBA switch, the other way
« Reply #7 on: 22:43, 06 August 12 »
Why are you linking pin 4 to the output via the 10K Resistor? Shouldn't this be tied to the 5V rail instead?
Where ? in picture or diagram ?
Look tied to 5V on both.

And is the 2.2K really necessary? You could probably link it directly. Nice solution though, I think I'll use it on my 6128 too :)
2.2k is required for the XOR open collector output. Driven input may go high by itself but not in a controlled way.

Offline Bryce

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Re: ABBA switch, the other way
« Reply #8 on: 22:47, 06 August 12 »
Doh, forget it, I was referring to the schematic, but I followed the track wrong, didn't realise that that IS the 5V rail. I should stop reading schematics after the 4th beer :D

Bryce.

Offline Audronic

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Re: ABBA switch, the other way
« Reply #9 on: 00:17, 21 January 16 »
@gerald

Do you have picture of where you mounted the Switch Please

Oops Found it

Thanks    Ray
« Last Edit: 04:16, 21 January 16 by Audronic »
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Offline xesrjb

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Re: ABBA switch, the other way
« Reply #10 on: 11:16, 23 May 21 »


When the switch is closed, the internal drive is A
When the switch is open, the internal drive is B



What does it mean exactly?


Switch is closed-> internal is A and external is B
Switch is open-> internal is B and external is A


xesrjb
„The pellet with the poison's in the vessel with the pestle; the chalice from the palace has the brew that is true.“

Offline gerald

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Re: ABBA switch, the other way
« Reply #11 on: 16:49, 24 May 21 »

What does it mean exactly?


Switch is closed-> internal is A and external is B
Switch is open-> internal is B and external is A


xesrjb
Yes !

Offline xesrjb

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Re: ABBA switch, the other way
« Reply #12 on: 18:04, 24 May 21 »
Best solution ever seen, I will try...


xesrjb
„The pellet with the poison's in the vessel with the pestle; the chalice from the palace has the brew that is true.“