Acorn Computers Limited Winchester disk 110, Service Manual, Page: 27 / 43

The REQ/ACK handshake operates only during data transfer. If the hardware for
this handshake is faulty then there can be no data transfer.
7.4 Bus lines
When the buses are not being asserted either by the SCSI interface or the host
microcomputer, ie in the bus free phase, all bus lines will float according to
the values of their terminating resistors.
Measure the voltage of each bus line in turn and make sure that none of them
is stuck at +5V, which would indicate a short circuit, or at 0V which would
indicate either that one of the buffers was enabled or that there was a short
circuit. The correct voltages are as follows:
1 MHz expansion bus D0 to D7 and A0 to A7 should all be 2.5V.
SCSI DB0 to DB7 should all be 3V.
If, for example, D0 to D7 are all 0V or a mixture of 0V and 2.5V then one of
the buffers is probably enabled.
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