Dr Jefyll wrote:
TTL Compatible... NOT! ( modern WDC CPU's )
In this thread I'd like to talk about the advisability of mixing modern WDC CPU's with RAM's, ROM's or PLD's etc whose output specifications only guarantee TTL voltage levels.
I honestly don't think it's much of an issue. I know I will more than likely, for the sake of future visits to this topic, be ignored and the argument will entertain us forever. So be it.
Bold assertion warning: If the devices (ROM, RAM PLD) are CMOS, you will have no issue whatsoever.You have to consider a few things:
1) Spec Sheets are created for 2 reasons. A) (the lesser reason) To give engineering guidance for reliable design purposes. B) (by far the main reason) To prevent companies like TI from being raked through the legal coals when a cruise missile misses the bunker in Syria and hits the White House instead because one of their 54LS32's did not meet spec. Or something to that effect.
2) Many of the devices in question were released into a market where TTL compatibility was still paramount. As deployment environments changed these devices were taken off the market so there was never a need to update their specs. As an example. Those TTL voltage specs were given for TTL loads. You will see them all quoted of a 400 uA load. In modern CMOS environments a load of 50 uA would be considered high.
3) Back in 80's when I was a bench tech at a teleprinter manufacturing plant I had to do my hours in QA like all of us. More than 99.9% of the TTL devices met or exceeded their "typical" output rating under full load. ~3.4V So the great preponderance of these ancient TTL devices should work just fine.
4) Basically 100% of CMOS devices have the nice, simple stacked MOSFET outputs we are all familiar with. They all work the same. In fact, those guaranteed to work with TTL loads should be expected to be more robust than to those not guaranteed to work with TTL loads.
As a exhibit I have attached the datasheet for the ST Micro M27C1001. Look at the output specifications on page 13. They list guaranteed output voltages for TTL (400 uA) and CMOS (100 uA). These same specifications can be expected (dare I say relied on) for any CMOS device that is TTL capable. This is why, when we use these CMOS RAMs, ROMs and PLDs with CMOS CPUs, they work to rated speeds and beyond.
So, if you're a hobbyist building your next interesting gadget then go for it. If you get the 1 in 10000++ device that does not pass muster, then gleefully thumb-toss it into the trash and put another in it's place (if this indeed happens in your life span) and carry on.
However, if you work for HP (who are they now?) building the next gen heart-lung machine or for General Dynamics building the next stealth AI bomb, you might want to take other precautions.
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