On Jan 4, 2012, at 11:20 PM, Bill Sudbrink wrote:
Josh Dersch wrote:
How does the "phantom" line work? I
see references to this in some of
my S-100 manuals but I'm not clear how it works (who raises/lowers it,
when, etc...)
It can be a little bit "implementation dependent" but usually there are
cards that assert phantom (it is active low, there has to be a pull up
on it somewhere (the CPU card I think)) and cards that "honor" phantom.
When an address is presented to the bus, any phantom asserting card that
whishes to override that address pulls the phantom line low. A phantom
honoring card that would normally respond to the address will instead
suppress any activity. Note that this system does not prevent multiple
phantom asserting or phantom honoring cards from stepping on each other.
This sounds a lot like the INHIBIT line on the Apple II bus. Is that roughly analogous?
- Dave