Acorn BBC Master 128, Service Manual

Page: 47 / 87

Master Series Service Manual
5 Is the SERPR0C getting a 1.23MHz timing reference, IC48
pin25? This signal could come from either one of two
sources -
a) A 74LS169 counter.
b) The chroma chip.
Whichever it SH0ULD come from is dependent on the build standard of
the computer. In any case, the SERPROC should get this signal from
some source. Check that it is the correct frequency.
6 Is the -5V supply correctly connected (and working) to the RS423
transmitter chip IC50 pin 8?
7 Are either CTS (IC48 pin 20) or RTS (IC48 pin 23) stuck high or low?
8 If all else fails, check all remaining connections to the ACIA and
SERPROC.
7.6.2 Will only receive data.
1 Does the remote computer use different transmit and receive rates?
If so check that your computer has the equivalent receive and transmit
respectively.
2 Check 3 6 and 7 as above.
3 Check that data going into the line driver IC50 comes out of it. The
driver could be faulty otherwise.
4 Check that the Tx CLK going into the ACIA is correct, IC45
pin 4. 7.6.3 Will only transmit data.
1 Does the remote computer use different transmit and receive rates?
If so check that your computer has the equivalent receive and transmit
respectively.
2 Check 3 6 and 7 as above.
3 Check that data going into the line receiver IC51 comes out of it.
4 Check that the Rx CLK going into the ACIA is correct, 1C45
pin 3. 7.7 Cassette interface
7.7.1 Will not load or save data.
1 Check the cable and data recorder (with another computer if
possible).
2 Check that the ACIA and SERPR0C have correct (i.e. no open or short
circuit) connections on all pins.
3 Check that the back panel connector has no dry joints or short
circuits between it and the interface circuitry.
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