Can I build with HC rather than HCT series ICs ?

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thwill
Posts: 4
Joined: 13 May 2019, 19:16

Can I build with HC rather than HCT series ICs ?

Post by thwill »

Hello folks,

First time poster, though I've had the kit unassembled for at least 2 years ;-)

A friend who in theory knows what he is talking about suggested that if I build my Gigatron using HC rather than HCT series ICs then it should be "more reliable".

This sounds like the sort of thing that will have been brought up before, but my google-fu is failing me, I've found this:

viewtopic.php?f=4&t=134&p=867&hilit=74HC+vs+74hct#p867

but I suspect there may be other posts even more relevant/specific.

Can anyone help ?

Best wishes,

Tom
at67
Posts: 435
Joined: 14 May 2018, 08:29

Re: Can I build with HC rather than HCT series ICs ?

Post by at67 »

In theory if you replace ALL the chips with pin equivalent HC versions you should get better noise immunity.

The problem is I don't think you can replace every chip with an HC equivalent and as soon as you start mixing TTL logic levels with HC logic levels you are invariably going to face problems.

I haven't heard of anyone running into reliability or noise immunity issues with any of the TTL logic level families for the Gigatron, so I would suggest that HC probably isn't worth the effort best case, worst case you'll have a non functioning board.

P.S. Welcome aboard/back.
Hans61
Posts: 48
Joined: 29 Dec 2020, 16:15
Location: Saxonia
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Re: Can I build with HC rather than HCT series ICs ?

Post by Hans61 »

The clock generator U1 (74HTC04) does not work with the 74HC type. With the rest it should be possible. I think so.
klf
Posts: 15
Joined: 10 Jan 2019, 22:48

Re: Can I build with HC rather than HCT series ICs ?

Post by klf »

My Gigatron is a wild mixture of LS, HC and HCT, whatever was available.

You have to keep in mind that HCT can be driven by LS, HC and HCT, but HC can only be driven by HC and HCT but not LS. The logic high output level of LS is too low for HC.
thwill
Posts: 4
Joined: 13 May 2019, 19:16

Re: Can I build with HC rather than HCT series ICs ?

Post by thwill »

Thank you for the answers.

I think my friend's thought was that if HC series were used throughout then there would be no issue with mixed logic levels and you would just get the advantage of the better noise immunity. He said that on his *brief* inspection he couldn't see anything that specifically depended on the use of TTL logic levels.

My friend seemed bemused that someone had the time and genius (his words) to design this and then the idiosyncracy to build it out of HCT chips and wondered whether the designers and kit suppliers were completely different people; I know they are not, or at least not when I bought my kit.

Best wishes,

Tom
Sugarplum
Posts: 43
Joined: 30 Sep 2020, 22:19

Re: Can I build with HC rather than HCT series ICs ?

Post by Sugarplum »

Yeah, the oscillator is about the only thing that is particularly picky. Most other inverter chips for that either won't oscillate (at least not in that configuration) or don't exist. When you go toward smaller packages, you find fewer available chips. Someone else noticed better stability with HCT rather than F for the IR and DR registers. But most of the design isn't picky.
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