IBM 3270 Maintenance Manual page 213

1.
Set the Power switch on the system unit (and
expansion unit, if attached) to Off for 5
seconds.
2. Set the Power switch on the expansion unit
(if attached) and the system unit to On.
3. Check that the voltage at pin 12 on the signal
cable connector decreases from approximately
5.0 Vdc at the start of POST, to
approximately 0 Vdc during POST.
DID THE VOLTAGE DECREASE TO
APPROXIMATELY 0 Vdc DURING POST?
NO Go to page 3-600-34.
YES Replace the diskette drive (see Section 5,
"RemovaI/R~placement
and Adjustments").
3-600-14 Maintenance Information