Acorn BBC Master 128, Service Manual

Page: 49 / 87

Master Series Service Manual
7.8 Disc drive interface
7.8.1 Refuses to work at all.
1 Check that both the signal connector and power connector are
securely pushed home.
2 Try another disc drive if possible, to eliminate the chance of a
fault in this.
3 Try the disc drive with another computer, if possible.
4 Check that the 1770/1772 and 74LS174 have valid logic 0 levels at
their device select inputs (IC3 pin 1 and IC4 pin 1) and that all data
and address bus connections are present, with no open or short
circuits. If either IC is not being selected then check the outputs on
the IO controller, IC15 pin 1 (FDCON) and pin 4 (FDC). If these
signals are not available then check that the disc software is fitted
and is selected.
7.8.2 The drive select light comes on. You cannot read a catalogue.
1 Has the correct filing system for the disc been selected (i.e. ADFS
or DFS)?
2 Is the disc formatted?
3 Is the disc in the right way round?
4 Is the Read Data input to the 1770/1772 (IC3 pin 19) wiggling when
you attempt to read the disc? If not then check the signal cable and
if necessary check that data is getting onto it from the disc drive.
5 Are the pull-up resistors (R1 R3 R5 and R6) on the 1770/1772 disc
inputs fitted and correct?
6 Is the 8MHz input to the 1770/1772 correct? Check that it is
available at IC3 pin 18. If not then trace it back to IC19 pin 4 and
IC42 pin 7.
7 Is the 1770/1772 DRQ line (IC3 pin 27) normally low but going high
once every 32us (when then disc interface has been asked to read, say,
a catalogue)? If not then check that this line is not shorted to power
or ground and is connected to the IO Controller (IC15 pin 5).
8 Check that the DRQ transitions (IC3 pin 27) appear logically
inverted at the NMI pin on the CPU (IC14 pin 6).
9 Check that the 1770/1772 INTRQ output (IC3 pin 28) produces a high
going pulse at the end of every command issued to the 1770/1772. If
the INTRQ line is shorted high it will cause the IO Controller to hold
the CPU NMI line low, thus masking DRQ transitions.
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