Microwriter hard reset?

Tony Duell ard at p850ug1.demon.co.uk
Mon Jan 2 06:14:33 CST 2006


> 
> Tony Duell wrote:
> > Does anyone have a user manual for a Microwriter MW4 chording keyboard-thing?
> 
> Somewhere, I think I do.  I'll have to search for it.

I guess I should put this project on hold until I get that sort of 
information. It's a waste of time looking for a fault that's not there.


> 
> > I have one of these
> > devices which I am currently running off my bench supply (4.8V) in place 
> > of the NiCd.
> 
> As you've realised, this is another one of those devices where the
> voltage "regulation" is done by the NiCds.

Yes. But the fuse (0.5A picofuse) is in series with the battery. So if it 
opens, the battery is out-of-circuit, but the charger still tries to 
power the rest of the machine. There is a zener diode (near the battery 
connector on the PCB) that acts as a regulator under those conditions, I 
am not sure what the voltage of it is. It is directly in parallel with 
the battery.

Another trap for the unwary is the power switch circuit. There is only 
one +5V line (connected to the +ve side of the battery), there are 2 
grounds, one of the always-on chips (RAM, and the power switch control 
NAND gate chip), the other for everything else. The power switching 
components -- the 2 VMOS FETs in parallel near the battery connector -- 
connect these ground rails together when the thing is on.

This is a somewhat neat trick in that it means that all the enable lines 
on the RAM are pulled to the +ve supply when you power down, in other 
words the RAM is disabled as you'd want it to be. And I suspect it 
simplifies driving the MOSFETs too. But it confused me for a few minutes...

As regards the chips in there, the CPU, ROM, and RAM are obvious. The 
PQFP thing under the display is, of course, the display driver. Alongside 
it is an empty 24 pin socket, this seems to take a 2716 character 
generator chip for non-English models. There's a 4555 decoder (RAM select 
and I/O select), a 4001 (mostly for I/O enalbes), a 4054 (latch with 
level shigted outputs), used for RS232 and cassette out, a dual 
comparator (cassette in and battery OK), a 4572 (4 NOTs, a NAND and a 
NOR), glue logic, a triple 3-input NAND (power switch bistable, RAM 
enable) and not a lot else.

The RAM is either 4 off 6116s, or 2 6264s in the sockets nearest the 
EPROM. There's a set of links in the middle of the board, you fit every 
other link, either starting at one end or the other depending on which 
sort of RAMs you have.

[Yes, I do have a schematic]

-tony



More information about the cctalk mailing list