Detecting the presence of a device on a computer system bus by altering the bus termination

DETAILED DESCRIPTION It is simple to detect when an ISA board drives values onto the data bus different from the bus' undriven state--the I/O or memory read returns a value other than the undriven data bus value 0 FFh on a pulled-up data bus, and 00 h on a pulled-down data bus.
The apparatus and method according to the invention, however, go further.
If a particular I/O or memory read returns a data bus value of 0 FFh, or whatever the undriven data bus value is, the apparatus and method according to the invention then performs a read cycle with the data bus pulled to its normally driven state.
If a read from the address under test still returns the normally undriven value, that indicates the slot under test contains a card that responds to the address under test.
Further, the apparatus and method according to the invention monitors the state of the data bus with at least one comparator.
This comparator permits detection of a card's response even when the level driven onto the data bus is not sufficient to drive standard logic devices to a particular state.