Re: Question about 74HCT and 74HC logic ICs

On Sun, 16 Sep 2007 00:30:54 GMT, Kyle J Cardoza
<admin@xxxxxxxxxxxxxxx> wrote:

Probably a stupid newbie question, but I've got a schematic for an EEPROM
programmer, and it calls for 74HCT series logic ICs. Is it going to cause
a problem if I use 74HC series ICs instead? Some of the 74HCT series are
proving hard for me to find, but I found 74HC series of the same numbers
easily, so I wondered if they could be substituted.

The HC parts have input switching thresholds around half the supply
voltage (2.5 volts for a 5 volt supply) while the HCT part have the
threshold lowered somewhat to better match bipolar (LS) TTL output
levels which are only guaranteed to be above 2.4 volts for a high.

If you add pull-up resistors of 5.1K or so to Vcc on all inputs to the
HC devices that should ensure that the signals actually rise above the
2.5 volt threshold for a logic 1.


If it matters, the EEPROM programmer I'm working on connects via PC
parallel port.

Thanks in advance.

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