IC replacement question

dahnz

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Hi all

In my ongoing attempt to fix my Williams games, I'm buying some spare ICs, but need to know if I can replace, say, a 7400 quad 2-input NAND with either a 74HC00 or a 74AS00 and a 7411 triple 3-input AND with a 74LS11. The LS, HC, and AS family types are the ones easily available here in NZ. Am I right in assuming I should go for the AS and LS over the HC or will they all work?

Many thanks, Dominic
 
Replacement ICs

DAHNZ,

You can PROBABLY use the 74AS chips... Almost surely the 74LS chips... but not the 74HC chips. The HC chips use CMOS levels and will either not work or eventually burn-up themselves or the other chips they are connected to... unless you make lots of modifications. The LS chips use less power but are almost the same, and will work in most cases. The AS/ALS chips are faster and use less power and are quite different, and may work in some cases. The HCT chips guarantee compatibility of input and output levels but are vastly different inside, only way to tell is to try.
The original circuitry sometimes relied upon the propagation delay of a certain chip or the input and/or output impedance to do some 'creative' timing, which could cause it not to work with anything other than the original part.
Best bet is try the 74LS first… then try the 74AS/ALS.
Here is a list:
74XXX Original TTL technology
74SXXX TTL employing Schottky transistors. Better pulse shape
74LSXXX TTL employing Schottky transistors. Less power consumption
74ASXXX Advanced Schottky TTL. Twice as fast as the "S" series
74ALSXXX Advanced Schottky TTL. Less power consumption
74FXXX Fast TTL (between 74AS and 74ALS)
74HCXXX High-speed CMOS transistors. For use with CMOS-only circuits
74HCTXXX High-speed CMOS with TTL-compatible logic levels
74VHCXXX Very high-speed CMOS
74VHCTXXX Very high-speed CMOS with TTL compatibility

Kenneth
 
Hi all

In my ongoing attempt to fix my Williams games, I'm buying some spare ICs, but need to know if I can replace, say, a 7400 quad 2-input NAND with either a 74HC00 or a 74AS00 and a 7411 triple 3-input AND with a 74LS11. The LS, HC, and AS family types are the ones easily available here in NZ. Am I right in assuming I should go for the AS and LS over the HC or will they all work?

Many thanks, Dominic


You have to be real careful when crossing between families.
The 74HCxx parts typically won't sub for anything other than the old 74Cxx parts.

Sometimes you can sub a 74LSxx part for a plain 74xx but not always. Most of the plain 74xx parts were slow but had high current drive on the output. The 74LSxx and 74HCTxx parts are faster and consume less power BUT they have a greatly diminshed output current capability. There are several pinball machine examples where 74xx parts are used and 74LSxx or 74HCTxx parts should not be used (I.e. 7432 used on switch matrix of Gottlieb System 80). The only way you can be sure is to evaluate the circuit - add up both high and low load's and see if they are supported by the part you wish to use as a sub.

Usually you can use 74Fxx, 74Sxx and 74ASxx in place of 74xx. These parts typically have the high current outputs. Downside is they are faster and more power hungry - faster can result in noisy signals.

Sometimes, the design relies on the 74xx's input impedance for proper operation. For example - the 7404 clock circuit on Gottlieb System 80's (again). You can use an LS part there but should never use an HCT part there. The HCT part has a high input impedance and may not work there... and if it does, it wouldn't be reliable.

So, to make a long answer short -- the answer is 'it depends on the exact method the IC is used as to whether or not the subs will work.'

Ed
 
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