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LUNA framebuffer mod for LCDs without Sync on Green support
> Framebuffer mod to connect the ancient LUNA to modern LCD:
> The original LUNA requires a "Sync-on-Green" display.
> However nowadays few LCDs support it so I modified the framebuffer
> to output H-sync and V-sync and disable Sync signals on Green line.
I received an inquiry about this framebuffer mod from a LUNA user,
so I'd like to note how I modified my LUNAs.
DISCLAIMER: there is no warranty. Try it ON YOUR OWN RISK :-)
There are many articles how to "separate" H/V-sync signals from
the Sync-on-Green line (for PlayStation2 Linux kit for example).
However, unfortunately this method doesn't always work on LUNA
because LUNA's compsite sync signal mixed on the Green signal
line is a bit strange.
Instead, I pickup H-sync and V-sync signals from CRTC chip
on the framebuffer and disable composite signal mixture
on the RAMDAC chip.
2. Sync signals on LUNA
The "Normal" composite signals on Green line should be as the following:
(this is green signal of DEC 3000/300 sfb(4))
On the other hand, LUNA (and some hp300 models) outputs the following
This means the h-sync signal stops during v-sync output period.
Most modern LCDs detects this short interrupt as "lost signal"
and doesn't display images properly.
Anyway, composite sync signal is generated from raw V/H-sync,
so it's much easier to use these raw signals directly than
separating mixed signals from green line.
3. Framebuffer mods
The necessary modifications are:
(1) disable "mixing composite sync onto green line" on RAMDAC
(2) pickup H/V-sync signals from CRTC via buffer
(3) pickup R/G/B signals from around BNC connectors
LUNA's 1bpp and 4bpp framebuffers use
HD6445P CRTC (40pin DIP) and Bt454 RAMDAC (44pin PLCC):
Package pin layout:
8bpp framebuffer uses
HD6445CP CRTC (44pin PLCC) and Bt458 RAMDAC (84pin PLCC):
Package pin layout:
3-(1) Disable "mixing composite sync onto green line" on RAMDAC
The composite sync signal generated by several TTL is connected
to "SYNC" pins of Bt454 (23pin) and Bt458 (58pin) to generate
sync-on-green signal. We have to pulldown this SYNC pin
to disable sync-on-green.
Instead of cutting pattern on PCBs, we can simply de-solder
and bend output pins of TTL that generates composite sync
from H/V-sync signals.
On 1/4bpp framebuffer the output is IC61 74ALS175 11pin:
(note my tweet says 10pin but it's typo)
On 8bpp framebuffer it's IC65 74ALS175 10pin:
You can simply connect the sync signal pattern to ground,
but it may be safer to use a proper pulldown registers.
3-(2) Pickup H/V-sync signals from CRTC via buffer
On 1/4bpp framebuffer H-sync and V-sync are connected to
IC52 74ALS32 14 pin and 15pin:
On 8bpp framebuffer H-sync and V-sync are connected to
IC65 74ALS175 4 pin and 5 pin:
(check which is V or H by your circuit tester)
I put 74HC04 as buffers to avoid ESD etc. from connector to CRTC chips,
though it's also better more LCR parts for proper protection..
(these were just my hobby mod ;-p)
I put the HC04 on other TTLs and only two invertors are necessary
for H/V signals. All unused pins should be connected to GND.
Inverted H/V-sync signals via HC04 are connected to VGA connector:
Modern LCDs recognize both polarities of sync signals, so
we don't have to consider how many times we should invert them :-)
3-(3) Pickup Red/Green/Blue signals from around BNC connectors
Sync signal on green line is already disabled so it's enough
to pickup RGB signals from pattern around BNC connectors:
(It's also better to use shielded cables but again it's my hobby mod)
On the 1bpp board, only green line can be connected to VGA
so you will get "green on black" monochrome screen.
You can also have circuits to distribute the green signal
to red and blue lines using some video buffers, but it's
4. Prepare backpanel for VGA connector
It's a simple sheet metal work. Aluminum sheet may be easy to work
using a drill and a metal file.
5. Assemble and check
It's also strongly recommended to check all connections
before powerup and tests.
(Sorry I didn't prepare schematics for these mod)
Again, there is no warranty on these descriptions and
there is a risk to break your favorite LUNA hardware during mod.
I strongly assume that you have enough digital electronics knowlege
and experience of such soldering work. Try it on your own risk.
Note there is another simply solution:
"Search displays that support sync-on-green."
There are several database for PlayStation2 Linux:
(though this might not work as expected due to the incomplete
composite sync signal on LUNA as described above)
Anyway, I hope this article will help more LUNAs to get working again.
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