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18.21 Lacescan
Lacescan Bildschirmerweiterung
Patching PCB traces LaceScan
Use a good multimeter set to a low-resistance range to verify
connections described below. NB: Values should be less than 1 ohm to
be acceptable.
Start by locating the MFP, a 48 pin DIL IC designated MC68901.
Pin 20 of the MFP (TBI = Timer B Input) is the original DE signal,
and must remain so even after installing the AutoLace circuit.
Next locate the shifter chip, a 40 pin DIL IC which is often
housed inside a metal shielding box on a standard ST/STFM. On Mega
ST's however there is no such box, and the shifter is usually
somewhere beneath PSU. Do not confuse the shifter with the other 40
pin chips (PSG & DMA).
Pin 37 of the shifter is the DE signal, which should be replaced
by the new signal generated by the AutoLace circuit.
Verify connection between shifter pin 37 and MFP pin 20.
The DE signal is created by the GLUE, a 68 or 80 pin PLCC (square)
IC, and sent to the MFP (see above), the shifter chip (see above), as
well as to the MMU which is also a 68 or 80 pin PLCC IC.
DE is generated at pin 33 of an 80 pin GLUE, but at pin 39 of a 68
pin GLUE.
DE is connected to pin 53 of an 80 pin MMU, but to pin 52 of a 68
pin MMU.
Verify connection between MFP pin 20 and the relevant pin of your
GLUE. Verify connection between MFP pin 20 and the relevant pin of
your MMU. These two verifications also help in identifying GLUE &
MMU.
Search the board visually to find a trace connecting DE signal of
GLUE to shifter and MMU.
Carefully scrape away the lacquer covering the trace (don't cut
trace!!!), and verify that it is connected to the DE pins of the four
chips.
Carefully cut away a short segment of the trace to break the
connection, leaving sufficient trace length to each side of the cut
for soldering. Scrape away the lacquer at each side of the cut and
apply some solder, to prevent copper oxidization to make future
soldering easy.
Verify that DE from the GLUE is no longer connected to DE of
shifter & MMU.
If DE of MFP is still connected to GLUE (and not to shifter &
MMU) then you have been lucky, and need not modify any more DE traces.
In the opposite case however, you must also cut the trace connecting
the MFP from shifter & MMU, and make a new connection from GLUE to
MFP.
In some rare cases the original trace reaches the chips in such
order that it will take three cuts and two new connections to achieve
the necessary configuration which is [GLUE with MFP] and [MMU with
shifter].
If you are unable to achieve the correct DE configuration (and
verify it), the best thing you can do is to restore the original
connections, and let someone else (more experienced) handle the
installation for you.
So, in other words... :
And the connection points to those 3 others ICs mentioned, from
that pin should all remain connected together as before?
pin 52, "MMU" chip (CO25912-38)
Moving on...
The only other cut necessary is due to a design error in most
ST's.
The two ACIA (MC6850) chips have an output that is not used, and
which consequently should have been left disconnected, which has not
been done. Instead some numbskull has decided to connect these two
together, which serves no purpose at all, but does cause hazard of
overheating.
In order for AutoLace to function this short-circut must be
broken, by cutting the trace connecting pin 5 of each ACIA to the
other. The remaining trace leading to pin 5 of the ACIA used for
keyboard should be prepared for soldering as described above, since it
controls AutoLace.
Alternatively, you can instead cut off pin 5 of keyboard ACIA
close to PCB and bend up the remainder of the pin (max 90 degrees) for
soldering, but take great care not to break the pin close to IC body
(unsolderable).
Done! (This problem doesn't seem to be in my STe though, so this
is probably only an ST/F/M/Mega occurance). Any other "factory
errors" I could fix while I'm at it?
As for the pin 5 of the keyboard ACIA -the instructions don't
mention *which* of the two 6850 chips it is....
And finally:
The additional traces that should be prepared for soldering are:
HSync at pin 36 of 80 pin GLUE, but at pin 37 of 68 pin GLUE.
After preparing these, you should verify that all are connected as
intended.
You are then ready to install the AutoLace circuit.
Copyright © Robert Schaffner (doit@doitarchive.de) Letzte Aktualisierung am 23. Mai 2004 |
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