AFAIR LS can only drive one unit TTL load.
I may have some 7474, even of that vintage, if you cannot find any
anywhere else.
cheers,
Nigel
Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU
Amateur Radio, the origin of the open-source concept!
Skype: TILBURY2591
On 2022-05-14 13:48, Paul Koning via cctalk wrote:
On May 14, 2022, at 1:41 PM, John Robertson via cctalk<cctalk@classiccmp.org>
wrote:
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
74<something>74 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.
I know LS has less fanout, but is it not able to drive plain 74xx at all? That
doesn't sound right. If the circuit in question runs near the fanout spec of
plain 74 the yes, 74LS won't work.
Spec sheets and the actual schematic will give a definitive answer.
paul
