A computer system determines the presence of a device on the system bus that responds to I/O or memory reads by performing an I/O or memory read with data bus pulled to its normally undriven state. If the value returned is other than the data bus' normally undriven value, it is determined that a device...http://www.google.com/patents/US5469554?utm_source=gb-gplus-sharePatent US5469554 - Detecting the presence of a device on a computer system bus by altering the bus termination
