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Re: [N8VEM-S100:3024] Re: Advice on building S-100 IDE board V2 (74H04 vs 74HCT04?)

David, Many thanks for the advice,
I'll read your notes and  experiment a bit with the HCTs then!
Fortunately I have a bunch of CFs and other old IDE drives to compare and help me see if the HCT is working...

On Apr 3, 2014 10:36 PM, "David Fry" <dgf...@googlemail.com> wrote:
Hi Fabio,
I recently had some fun with this issue on my S-100 IDE V02 Card (with twin sockets)
Looking at the circuit for your V2 card the 74LS04 drives the ICS0, ISC1, IORDA, IOWRA & RESET lines on the IDE socket whereas the V02 card uses a 74LS03 to permit a 2 socket setup. The problem I noted in my card was that (as John has pointed out)  the logic levels dont hit the minimum 4v required
by the IDE specification, it would appear that the CF card itself loads the gate output enough to drag down the logic 1 levels to about 3.4v, this was observed on
an oscilloscope where on ejecting the CF card the logic 1 level jumped to the full 5v.
This effect may be CF card dependant, mine was an old 256MB CF card at the time. This led me to change my 74LS03 to a 74S03 and drop the value of RR1 from 680 ohms to 330 ohms to increase the drive capability. This rectified the issue on my card and I have had no problems since.
In your case, and looking at the datasheet I would suggest that a 74HCT04 should work as it is capable of driving upto +/-25mA
I have done some notes for the V02 card that you might like to read, they are in the N8VEM builders notes folder
David Fry
On Thursday, April 3, 2014 8:55:26 PM UTC+1, Fabio Battaglia wrote:
Hi everyone!
Today I got some old scrap boards/circuits and recovered enough components to build an IDE V2 card (one of those with the single IDE connector on them).
I've completed my build, except for a component which is leaving me some doubts: U4 is a 74LS04 on the silkscreen, but on the card's page (http://s100computers.com/My%20System%20Pages/IDE%20Board/My%20IDE%20Card.htm) it's written that it MUST be a 74S04 or 74H04.
On the V3 card description there it's mentioned that this component was required because the outputs reached a level high enough for the IDE specs, a level which was not attained with LS ICs. AFAIK both 74S and 74H are still TTL chips, wouldn't it make sense to replace the '04 with a 74HCT04 which surely reaches a level high enough to satisfy the requirements?
Ideas? Opinions?

I'm considering just trying and see what happens, but it'll take some time before I'm able to write 6502 code capable of using the IDE board.


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