512K Rom/Ram card

[Ronald Maas]

Hi,

Just looking at the schematic for this card. There are pulldown resistors on there. Some of 4.7K and some of 1K. As far as I can see they all serve the same purpose. I don't understand why they have different values.

Any ideas or am I missing the obvious ?

Thanks,

Ronald

[Mark T]

Suspect this was carried forward from a previous design using 74ls for address decoding. 1k would be the recomended pull down on the input of 74ls series logic. The schematic shows 74ls139, but I think spencer ships 74hct series.

They could probably all be increased to 10k for 74hct series.

These pull downs are only used until PAGE_EN is active and it doesn’t seem that bus capacitance would be a problem, so could even be increased further to 20k or more if current drain was a concern.