+AD4- The first thing I would do is to trace where the connections
+AD4- from the serial port go.
Opening it up I learned via the prominent
rectifier that the power is AC at least.
There's a big Toshiba TMP80C49P that looks like a 'keyboard'
MCU to me. Some minor things like Motorola SN74LS(05N,138N).
It's a standard microcontroller, intenral ROM. Look for the '8048' -- the
8049 is the same chip with more memory (and the 8050 has even more), the
'C' in the middle means a CMOS low-power part.
Nearest the SIO port are just a few resistors and two
transistors. At least one leg of the SIO goes directly to
Could be anything, including RS232 buffers....
You can convert RS232 levels to TTL using an NPN transistor, a diode (if
you're a purist -- connect it across the BE junction of the transistor to
prevent the latter breaking down on a -ve input) and 3 resistors. You can
do the reverse -- TTL to RS232 levels with a PNP transsitor and 3
resistors, assuming a -ve voltage line is available, and that a 5V
ouptu-high level is enough. One 'quick-n-dirty; trick I've seen is to use
a 79x05 as the power supply regulator (so the unregulated +ve line is the
5V line, the output of the regulator is the system 0V line), and to use
the unregulated input to the regulator, which will be 4 or 5V below
'ground' as the -ve supply to the RS232 transmitter.
the toshiba. Looks non-RS232 then... Whatsit 5V
instead of 15?
I would trace out all 3 connections from the serial connector, at least
as far as the microcontorller pins. Can't be that big a job....
+AD4- If one of them is at a -ve voltage, ...
'+ve' is positive...