max IEC bus length and loading?

We have been doing some robustness testing of the ZoomFloppy board. It
uses a 7406 for driving the lines in order to be able to sink a decent
amount of current to support multiple IEC devices on the bus.

http://www.root.org/~nate/c64/xum1541/

What is the defined worst-case IEC bus configuration that we should
support? I'd like a reasonable limit we can support while not adding too
much delay for users of one or two drives.

For example, Wolfgang Moser has 4 drives with a total bus length of 4
meters. The firmware works there mostly but he experiences a glitch
after running a continuous test loop for a few hours.

I'm looking for any specs for the limits of the IEC bus as supported by
Commodore. I have the IEC timing diagram, but it doesn't describe the
max bus load. You can infer the max current based on the fact that
drives have 1K pullup resistors to 5V. But we'd like something more
official.

I'm also interested in anecdotal evidence of how many people would use
this device with >2 drives. It is a USB microcontroller that supports
both serial and parallel transfers via the OpenCBM software. You can
also plug in multiple ZoomFloppy devices if you want to support more drives.

Thanks,
-- 
Nate


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