ABBA switch, the other way

Started by gerald, 19:37, 06 August 12

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gerald

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

IanS

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.

gerald

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.

gerald

Pictures !!!

Bottom side
[ You are not allowed to view this attachment ]
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
[ You are not allowed to view this 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.

IanS

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?)

gerald

Quote from: IanS on 21:12, 06 August 12
(What is the other board with the crystal on over the 8255?)

That's a PS2 keyboard adapter.

Bryce

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.

gerald

Quote from: Bryce 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?
Where ? in picture or diagram ?
Look tied to 5V on both.

Quote from: Bryce on 22:28, 06 August 12
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.

Bryce

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.

Audronic

#9
@gerald

Do you have picture of where you mounted the Switch Please

Oops Found it

Thanks    Ray
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

xesrjb

Quote from: gerald on 19:37, 06 August 12


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."

gerald

Quote from: xesrjb on 11:16, 23 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 !

xesrjb

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."

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