The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
awemawson:
Another day going cross-eyed but quite a bit of progress :thumbup:
I initially concentrated on the 'low battery alarm' circuitry which uses a dual precision comparator chip. Got it's wiring worked out but I couldn't for the life of me find where the output of the second part went. I couldn't trace ANY pcb tracks but it's not easy with SMDs ! Bright torches help shining from the other side, but not in this case. In the end I decided to butcher the original battery RAM card that was dissolved by the lithium juice prior to my ownership, and take the 8 pin SMD off the board. Hot air gun suitably applied only to reveal that despite the fact that it's inputs are connected, IT'S NOT USED :clap:
OK go back to chasing real bits not virtual ones. Next on my list to nail was the three pin SMD labelled '45'. The look up list for SMD's said it could be a pair of diodes, a single diode, a transistor, or a voltage regulator so not much help. In the event looking at the original card (where this chip had completely disappeared!) in fact only two pins had tracks attached so it could really only be a diode (probably schottky from my meter readings)
Time to try and identify the SMD device labelled 'AJ' - again the list said that it could be almost anything, so I decided to pull one off the scrapped board and try and test it. Fiddly things these SMD chips - breath and they skid all over the place. In the end I super-glued it to a bit of wood, pressed pins in adjacent to the connections, and biased them together with a Hellerman rubber sleeve. Didn't do me any good - my component tester had no idea what it was. So I've drawn up just a block with three connections for the circuit drawing hoping that it's identity might be revealed by context.
I still have quite a few 10K resistors to positively tie to places in the diagram - several are definitely pull ups for gate inputs from the outside world, but I'll save to joy of finding them for tomorrow!
jiihoo:
Watching with interest and learning from your reverse engineering techniques.
Tying all inputs of CMOS chips to a high or low level should be standard practice. Most CMOS chips (logic gates and comparators) will start oscillating if they have floating inputs, which can cause excessive current consumption and may interfere with adjacent circuitry. I assume Andrew knows this; this explanation is for others reading this thread :D
awemawson:
So with a flurry of activity this morning I think I've got all the components accounted for and correctly interconnected on the circuit diagram - phew that was quite a bit of labour and eyestrain!
Of the 48 pins on the edge connector only two are not used (C15 & C16)
I still don't know what the SMD chips 'AJ' actually do, but from the circuit you can see that they are turning on the PNP transistor 'SMD-DB' when 5 volts is coming in from the edge connector, and turning it off when it's not present allowing power to pass to the RAM chips normally from the incoming supply, and be powered via the diode '45-SMD' from the lithium battery when system power of 5 volts goes away.
By my calculations the 'Low Battery Alarm' (Pin A16) should be asserted when the volts of the 3.7 volt nominal battery fall to 2.8 volts.
Now I have the circuit I need to work out how to actually test it and hopefully fix what is wrong. The battery side should be pretty simple. Power up the card with 5 volts, replace the battery with a variable lab supply and see what happens when the voltage is lowered from 3.7.
However testing the rest isn't quite as simple. Simplistically all 16 address lines and 16 bi-directional data lines should be able to be toggled by applying a TTL signal so any 'stuck bits' could be found, but Id like to build a proper tester that writes to the ram and checks what is written. My first thought was something like an Arduino, counting up the address line in binary, and performing a write and a read but the number of input output pins gets large.
16 address, 16 data, plus Chip Select, Write Enable and Output Enable is 35 I/O lines - will an Arduino do this - I have no idea never having used one! I can cut the number down by 15 to 20 by building a 16 bit counter circuit for the address lines and just incrementing it but that limits me to only testing addresses sequentially which is not ideal.
. . .so comments please from all those Arduino experts out there !
Muzzerboy:
That battery / psu circuit with the 2 "AJ" devices sort of makes sense to some degree. In the circuit, the LH one looks to be an NPN transistor and the RH one appears to be a zener - but the same device can't do both functions. Or can it?
The LH device looks like pin 1 - base, pin 2 - emitter, pin 3 collector, which is a fairly normal configuration for a bipolar transistor. You can use the base-emitter of a bipolar transistor in reverse as a zener (typ around 6.8V breakdown?) or emitter-collector for that matter. I've seen a few circuits over the years where that's been done.
Trouble is, the supply voltage is only 5V, so you'd never get the base junction to avalanche. It seems to almost make sense - but not quite!
jiihoo:
The Arduino Mega 2560 would fit the bill. It is the Arduino that has the most I/O directly on board: 54 digital I/O pins. It costs around €35.
https://www.arduino.cc/en/pmwiki.php?n=Main/arduinoBoardMega2560 has the specs.
I only dabble in Arduino, meaning I have a few of the boards and have compiled some simple sketches a few years ago. More knowledgeable members might be able to recommend a more optimal solution like a smaller Arduino with a suitable shield card. In my opinion the Mega ain't too badly priced and it would be a single board solution, so with my limited knowledge that's what I'd pick.
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