prindle@nadc.arpa (02/20/87)
I recently posted this to comp.sys.cbm for the Commodore 128, but just realized
that it might be useful to the general microcomputer community. It is a
simple circuit which accepts RGBI input (with NTSC sync, such as produced by
a PC with Color Graphics Adapter, not EGA) and produces a monochrome composite
output with 16 grey levels (green, amber, whatever) suitable for use with a
standard NTSC monochrome monitor. Sync is not handled with great elegance, so
some monitors might show some flagging at the top, but this is not my exper-
ience. The schematic is included below.
Sincerely,
Frank Prindle
Prindle@NADC.arpa
-------------------------------------------------------------------------------
Digital RGBI to Monochrome (Gray Scale) Adapter (NTSC)
------------------------------------------------------
RGBI(DB9)
Connector
--------- (stolen from computer or separate supply)
* 100 DIODE +5 Volts
Pin 3 R>---/\/\/\/--->|----+ v
| 680 |
* 220 DIODE | +--/\/\/\/--+
Pin 4 G>---/\/\/\/--->|----+ | |
| | 2N2222 |
* 300 DIODE | 100 100 | |-----+
Pin 5 B>---/\/\/\/--->|----+----/\/\/\/--+--/\/\/\/--+-----|
| | |->---+---------->
* 240 47 | | | Video
Pin 6 I>---/\/\/\/---------+----/\/\/\/--+ | +-----+ Out
| | | To
| 33 | | 82 Monitor
+----/\/\/\/--+ | +--/\/\/\/--+---->
| | |
V | V
(gnd) | (gnd)
2N2222 |
100 |-------------------------+
Pin 8 H>---/\/\/\/---------| |
|->---+ |
| | Note: + denotes a
V | connection; there
(gnd) | are no crossovers
2N2222 | in this schematic.
100 |-------------------------+
Pin 9 V>---/\/\/\/---------|
|->---+
|
V
(gnd)
Pin 1 >------------------------------+ Note: Carry both grounds through
| and tie both to ground to avoid
Pin 2 >------------------------------+ signal degradation.
|
V
(gnd)
* These four resistors determine the gray scale. As specified, they provide
16 different gray levels, some of which are only barely different - red is
most heavily weighted, green next, and blue is lightly weighted. To allow
experimentation or customization of gray scale, mount these resistors on
a DIP socket or use 500 ohm trim-pots.
> Drawing schematics with ASCII characters is an imprecise art. If you don't
see three (3) transistors above, keep looking; they are there! I believe
the diodes I used were germanium - if you use silicon, the picture may be
dimmer overall unless you adjust the weighting resistor values.