Re: Replacement for a DEC 7474 Chip

2022-05-15 Thread Paul Koning via cctech 



> On May 15, 2022, at 2:51 PM, Chris Quayle via cctech  
> wrote:
> 
> ...
> Original 74 series TTL can still be found, Ebay often, new old stock
> and other surplus vendors.
> 
> Would need to take care replacing with later ttl series, as the gate
> delay, switching times and fanout probably won't match. Always use the
> original series if you can...

That's sensible advice.  When you can't, faster devices with adequate fanout 
should work.  The exception would be in circuits with hairy timing, where the 
design relies on the actual stage delays of the components.  Most people don't 
build that way, for good reason.  Seymour Cray is the best known exception; for 
example, the CDC 6600 design is very thoroughly dependent on the circuit and 
wire delays, and either slower OR faster will break things.  And actually, it 
isn't clear to me why the thing ever works; the design files clearly 
demonstrate clashing signals that somehow apparently don't show up in reality.

paul

Re: Replacement for a DEC 7474 Chip

2022-05-15 Thread Chris Quayle via cctech 

On 05/15/22 18:00, cctech-requ...@classiccmp.org wrote:

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Today's Topics:

1. Replacement for a DEC 7474 Chip (Rob Jarratt)
    2. Re: Replacement for a DEC 7474 Chip (John Robertson)
    3. Re: Replacement for a DEC 7474 Chip (Paul Koning)
    4. Re: Replacement for a DEC 7474 Chip (Nigel Johnson Ham)
    5. Re: Replacement for a DEC 7474 Chip (ben)
    6. Re: Replacement for a DEC 7474 Chip (Chuck Guzis)
    7. Re: Replacement for a DEC 7474 Chip (ben)
    8. Re: Replacement for a DEC 7474 Chip (dwight)
    9. Re: Replacement for a DEC 7474 Chip (Chris Zach)
   10. RE: Replacement for a DEC 7474 Chip (Rob Jarratt)
   11. Re: Replacement for a DEC 7474 Chip (Eric Smith)
   12. Re: Replacement for a DEC 7474 Chip (Eric Smith)
   13. Re: Replacement for a DEC 7474 Chip (Chuck Guzis)


--

Message: 1
Date: Sat, 14 May 2022 18:11:32 +0100
From: "Rob Jarratt"
To: "General Discussion: On-Topic and Off-Topic Posts"

Subject: Replacement for a DEC 7474 Chip
Message-ID:<007b01d867b5$a8c90f00$fa5b2d00$@ntlworld.com>
Content-Type: text/plain;   charset="us-ascii"

Hello,



I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
7474 D flip flop. The problem is I don't know which modern series
would be the best one to replace it with. I am sure I have seen a list
somewhere of modern equivalents for some DEC chip numbers, but I can't
remember where.



If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the
schematic (p157 of the PDF).



Picture of the failed chip here:
https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg



Can anyone tell me what the best modern equivalent is likely to be?



Thanks



Rob



--

Message: 2
Date: Sat, 14 May 2022 10:41:34 -0700
From: John Robertson
To: Rob Jarratt via cctalk
Subject: Re: Replacement for a DEC 7474 Chip
Message-ID:
Content-Type: text/plain; charset=UTF-8; format=flowed

On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote:

Hello,

I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
7474 D flip flop. The problem is I don't know which modern series
would be the best one to replace it with. I am sure I have seen a list
somewhere of modern equivalents for some DEC chip numbers, but I can't
remember where.

If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the
schematic (p157 of the PDF).

Picture of the failed chip here:
https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg

Can anyone tell me what the best modern equivalent is likely to be?

Thanks

Rob


You are stuck with using an original 7474 family assuming this is
driving other early TTL. 74LS74, and others simply don't have the drive
capability to work. You can use 74S74 or 74F74 as they have the same
output current, the "S" is Schottky, and the other is "F"aster.

I find that https:///unicornelectronics.com is a reliable source of TTL.

John :-#)#



Original 74 series TTL can still be found, Ebay often, new old stock
and other surplus vendors.

Would need to take care replacing with later ttl series, as the gate
delay, switching times and fanout probably won't match. Always use the
original series if you can...

Chris