[Coco] Re: [Color Computer] Interfacing question.

[Phill Harvey-Smith]

Gene Heskett wrote:
> On Monday 21 March 2005 13:27, Phill Harvey-Smith wrote:
> 
>>Dunno if this is a silly question but here goes....I'm trying to
>>re-implement the RS dos controler using PLD logic chips (22v10 etc),
>>and have run accross a little problem. I have a 22v10, that does
>>the address decodde for the FDC and for the drive/density/halt
>>enable. This chip also has the latch for the halt enable. However I
>>find that this latch will not work with A3 connected UNLESS I
>>install a pull-up on this line.
> 
> 
> Have you got good scope Phill?  I'm wondering if somehow your 6809 has 
> been banged around via pulling a game cart when things were hot, or 
> worse yet plugging one in hot, which might have blown the internal 
> pullup transistor in that 6809. 

Scoped them this evening all seem to be reaching 5V, though some of them 
it's easier to see than others, guess it just depends what's on the lines :)
Odd thing is though if I keep scope probe on line when holding Reset I 
get 2V, this does seem a little strange to me.

>				 Checkable if its socketed by trying 
> another 6809, or better yet, a 6309 as its active pullups are quite 

Have tried both my CoCo 1 and my Dragon same results. I do have a couple 
of spare 6809s though, so I could indeed try changing one of them. Don't 
have a 6309 unfortunatly.

> strong and go *all* the way to the 5 volt rail, unlike the 6809.  It 
> is a pure CMOS chip, using only a few milliwatts of power at the 
> coco's clock frequencies.

May try andd get hold of a couple it is a pin compatible drop in isn't it ?

> Compare the height of the logic one on A3 with that of one of the 
> other address lines, for TTL compatibility it will need to hit around 
> 2.7 to 3 volts.
> You might check the expected logoc levels the 22v10 needs, maybe it 
> needs more logic 1 than the normal TTL specs do?  I don't personally 
> know on that point.

Yep, definiatly reaching this high, acording to the data sheet for the 
ATF22v10BQL that I am using anything above 2.0v should be seen as logic 1.

>>So I guess my question is, is there anything strange about A3 ? that
>>can cause this to happen. I am eventually planning to move over to
>>a CPLD, but I hopefully don't want to have the same trouble with
>>that.
> 
> Not that I'm aware of Phill.  A0 and A1 catch a bit more load than the 
> rest, but A3 really shouldn't have any more load on it that A4 thru 
> A15 unless something gone foobar.

Humm that's what I figured.

The really odd thing is that A3 is affecting my PLD even if it is 
connected to it but not used in the internal logic which is weird I have 
verified this by temproally disconnecting (power off of course) the pin, 
the damn thing works absolutely fine then, but of course I need A3 to 
select FF40/FF48 :)

Phill.


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