The subject says it all: Are pull-up resistors on the databus really mandatory on the MSX standard? Where can I find this info?
No they are not mandatory.
The usage of pull-up resistors depends on the application.
Pull-up resistors are mainly used to maintain voltage levels.
For example:
if you have a lot of signal degradation because of transporting signals across long cables
then you would use them and usually in combination with buffers and pull-down resistors.
as to where to find this info I would say the electronic bible....a book called:
"The Art of Electronics" by Horowitz & Hill
Everything you'd ever wanted to know about basic-electronics is in here.
On the MSX databus pull-ups would be present for several reasons:
To make sure that if no IC outputs data, the databus doesn't float. With 'float' meaning: voltage levels wandering up or down in semi-random manner. This is bad because it might cause circuitry that reads the databus, to switch internal circuitry more often than it otherwise would (=unnecessarily increases power consumption).
If an attempt is made to access non-present hardware, return a defined 'empty' value (FFh, all 1 bits). Good to have the same value returned always, in case some software is stupid enough to rely on that.
To make low -> high transitions a bit faster, which might help in cases where timing becomes critical. Think MSX stuffed with cartridges and/or internal expansions. With proper value & location, doubling as some sort of line termination (like marlon-B said).
None of these essential AFAIK, but all good reasons. If it isn't mentioned / required in any MSX standard documentation, that's probably because it would be implied - expected from any good circuit designer & therefore no need to explicitly require it. Btw: second that book recommendation - it's dense & many pages, but like an "all you ever wanted to know about electronics".
