ZX Interface I - under the hood

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tcat
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ZX Interface I - under the hood

Postby tcat » Fri Dec 08, 2017 8:55 am

Hi,

I would have normally posted to `World Of Spectrum' forum, but I got SSL handshake failed, so I hope may get some attention here as well.

I have recently recapped my Speccy also I1, now looking inside it, I would like to know the function of a transistor flying on top of ULA. Please see picture. Reading `ZX&I1' service manual does not give the answer.

i1-ula.JPG
Interface1 - ULA

Many thanks

Tomas


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Pr0f
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Re: ZX Interface I - under the hood

Postby Pr0f » Fri Dec 08, 2017 9:06 am

Is it possible to see what pins it's attached to?

I suspect it could be being used as a cheap and dirty inverter circuit, looks like it comes off the pin 8 of that ttl logic chip - which is a hex inverter, but what pin is it driving on the ULA - your service guide should tell you that, hopefully.

I have some info on the circuit back home, but at work at the moment, so couldn't tell you exactly what it's connected to and why.


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Bloodnok
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Re: ZX Interface I - under the hood

Postby Bloodnok » Fri Dec 08, 2017 9:18 am

I believe this is what the extra transistor (Q11) is for....

To quote from the service manual:

'When /IORQ is asserted from the Spectrum, the Spectrum's own ULA may inhibit the CPU clock for several
cycles. Transistor Q10 disables this action for any I/O transaction except for those during which A0 is low
(i.e. I/O transactions with Spectrum ULA). This transistor is duplicated inside the Spectrum in later issues.
Transistor Q11 is provided to speed up the propagation of IORQ from Z80 to Spectrum ULA. This is
necessary to overcome the extra capacitance on the processor bus and particularly on IORQGE.'

Brendan Alford has made a great job of reformatting the service manual. Check it out here:

https://spectrumforeveryone.com/wp-cont ... Manual.pdf

I found the details on page 4 under section 1.1.2 Address Decoder IC3. Earlier manuals had the description in a different section.

Charles


tcat
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Re: ZX Interface I - under the hood

Postby tcat » Fri Dec 08, 2017 9:27 am

Hi Prof,

Thank you. I have assembled it again. But it seems middle transistor leg (possibly its base), is attached to ULA pin 10? Perhaps service manual has some info, that I do not get. I have only a circuit diagram inside the manual of poorer quality.

Tomas


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Pr0f
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Re: ZX Interface I - under the hood

Postby Pr0f » Fri Dec 08, 2017 9:35 am

Middle leg isn't always base I'm afraid - depends on the transistor and sometimes on package option for the same transistor too.

If you can read the part number of the transistor, then you should be able to google for the datasheet - the pin outs are almost always shown as if viewing it from underneath. Have you seen the comment from bloodnok too - might be helpful to you :-)


tcat
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Re: ZX Interface I - under the hood

Postby tcat » Fri Dec 08, 2017 10:26 am

Hi Charles,

Your post was quicker than mine. I see Q11 transistor. My Speccy is Iss#3 so perhaps a later model.
I was confused when looking at ULA pin1 (with a dot) and its opposite pin 40 ~ with +5V VCC, this is where one transistor leg is fitted through a wire? Compared to the diagram, annotated by me.

Q11-IORQ-driver.png
ZX Interface I - Q11 transistor
Q11-IORQ-driver.png (8.7 KiB) Viewed 431 times

Correct?
Tomas



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