I must be searching for the wrong terms because Im sure I've seen it mentioned but I can't seem to find the info for the drive current of A0-A15 and D0-D7 of a 6502.

I tried to work it out from the datasheet, but.. well.. I'm too stupid

Could someone post the link to the topic, or just repeat it again

Also, if I connect (for example) A04 to 8 devices, but only one of those devices is enabled (maybe CE\ low or whatever), so the other 7 devices still count against the loading on the bus or are they considered high-impedance?

The reason I ask is that my system Im building will have 8 'add-on' slots for extra devices, and its possible that all 8 might have cards in them, although only one would be selected at a time. Would the other 7 still load down the buses even though they arent selected?

> Could someone post the link to the topic

http://www.6502.org/forum/viewtopic.php ... light=50ma

CMOS outputs will drive a ton of LSTTL inputs, but there's no reason to use LSTTL anymore since CMOS is just as fast or faster, lower-power, inexpensive, widely available, etc..

> so the other 7 devices still count against the loading on the bus or are they considered high-impedance?

With CMOS, it's pretty much the same thing as far as bus loading goes. A CMOS input takes virtually no DC current (just a tiny amount of leakage), but there's still the capacitance associated with it. At really low speeds, CMOS' fan-out capability is essentially unlimited. At the higher speeds, the capacitances will begin to take their toll. These capacitances are in the circuit boards and connectors, not just the ICs' inputs.

> Would the other 7 still load down the buses even though they arent selected

Yes

If you're going to be operating at speeds where the bus loading becomes a real issue, I would recommend keeping on the main board all the I/O ICs that are actually connected to the processor's own buses. That way you'll be able to run the clock speed a lot higher, even though other things have to go through the existing I/O ICs (65c22's or whatever). For example, if you want an LCD, connect it through a VIA. LCDs' processor interface is never capable of very high speeds anyway. If you want A/D and D/A converters, interface them through a VIA. A lot of I/O you'll want does not have to be super fast anyway, and interfacing through something like SPI or I²C bit-banged on a VIA is simple and takes a lot less wires, making it more likely that you'll live long enough to see your project through to completion!

Since I'm running my system at 1/2 Mhz, I thought I could just interface directly to the address/data busses (perhaps with buffering?)

The reason I asked about the drive current is that I am going to have 8 plug in slots to make playing around easier. All the slots will have A0-A4 connected to them, although only one slot would be active at a time - just wondering if having 8 cards on the A-bus will cause any adverse effects I need to be wary of. The A-lines would always be connected, but the IC's they are connected to would be disabled (except the one in use of course).

[quote="Sentient"]Since I'm running my system at 1/2 Mhz, I thought I could just interface directly to the address/data busses (perhaps with buffering?)
---------------8<-------------------------------------------------------
quote]

This idea has some interesting possibilities. If you are going to
keep the clock speed down, you could even use " old-fashioned " DB-25
RS232 connectors as your bus interface connectors. But, PC 8bit ISA
slot connectors are still widely availible and cheap, so that is probably
the way to go.

As for information on whether to buffer or not, I would say look
for schematics on the the IBM PC / Apple 2 or the good ole 8bit STD bus,
and you will get some good ideas. If you cannot find STD bus schematics,
email me privately and I may be able to dig something out.

- Regards

Wallace

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Now that I re-read that, might be a conusion - I meant 1 or 2 Mhz, but I guess that doesnt change the fact I can be 'rough-as-guts' on the circuitry so to speak.

The reason I'm going with the expansion ports isnt to give me more space. I just like the idea of expansion cards - in other words, I can so I am

I plan on using simple little pin headers/sockets for the expansion ports (something like a 10x2 header/socket or whatever requirement dictates) since the design is on veroboard (veraboard?!?versaboard - something like that).

All parts are CMOS (6502, 6522 etc) and the gates are HC.