On 15-8-2009 13:37, Orchid XP v8 wrote:
>>>> Good grief Andrew, 7400s, did you rob a museum?

> 
>> IIRC I think the specs said that TTL inputs would be at a logical 1 if 
>> there was
>> no connection. (Internal resistor to Vcc) 
> 
> Oh.
> 
> Well that would be a problem then. I was assuming it would be logic 0. o_O

Starting middle of September I'll be teaching digital technology in the 
first year of the University of Applied Science in Amsterdam. If you 
happen to be around, just drop in. I think I will do the 7400 in the 
week of Sept 21. ;)

As others have said, the original TTL series by Texas Instruments acted 
as if a one was present if left open. All other gates should do the same 
because of backwards compatibility. That includes the CMOS pin 
compatible replacements. I have actually seen rare default 0 gates but I 
would consider those faulty.

When designing the rule is not to leave inputs open. Not even for gates 
you don't use. One reason is that they could pick up interference which 
could result in unpredictable behaviour. For CMOS gates it could also 
result in useless power consumption. In any case the inputs are often 
next to one another like in the 7400. It is just as easy to wire them 
together (connecting them to ground (for NOR gates) or the positive rail 
(for NAND gates) is often more complicated).

Oh, and if you want to build your own computer, I'd suggest using also 
other components apart from the 7400.

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