semi-OT: text lites

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Fri Apr 30 17:53:00 2004

>
> +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?

Could be...

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, ...
>
> +ACI-ve+ACI-... Eh?

Negative.
'+ve' is positive...

-tony
Received on Fri Apr 30 2004 - 17:53:00 BST

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