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General Category => Amstrad CPC hardware => Topic started by: gerald on 19:37, 06 August 12

Title: ABBA switch, the other way
Post by: gerald 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
Title: Re: ABBA switch, the other way
Post by: IanS 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.
Title: Re: ABBA switch, the other way
Post by: gerald 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.
Title: Re: ABBA switch, the other way
Post by: gerald on 21:08, 06 August 12
Pictures !!!

Bottom side
 [ You are not allowed to view attachments ]
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 attachments ]
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.
Title: Re: ABBA switch, the other way
Post by: IanS 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?)
Title: Re: ABBA switch, the other way
Post by: gerald on 21:23, 06 August 12
(What is the other board with the crystal on over the 8255?)

That's a PS2 keyboard adapter.
Title: Re: ABBA switch, the other way
Post by: 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? 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.
Title: Re: ABBA switch, the other way
Post by: gerald 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.
Title: Re: ABBA switch, the other way
Post by: Bryce 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.
Title: Re: ABBA switch, the other way
Post by: Audronic on 00:17, 21 January 16
@gerald (http://www.cpcwiki.eu/forum/index.php?action=profile;u=250)

Do you have picture of where you mounted the Switch Please

Oops Found it

Thanks    Ray
Title: Re: ABBA switch, the other way
Post by: xesrjb 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
Title: Re: ABBA switch, the other way
Post by: gerald 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 !
Title: Re: ABBA switch, the other way
Post by: xesrjb on 18:04, 24 May 21
Best solution ever seen, I will try...


xesrjb