Programming new software for a video titler?

[GlueLogic]

I recently took apart a VTG228 video titler to remove the old backup battery and dump the ROM contents (available here).

In general, old video titlers are regularly available for around $15-$20 on eBay (at least here in the UK), as there's not much use for them in the modern age aside from for making Local58-style horror videos.

Inside, the VTG228 is essentially an entire 8-bit microcomputer with some extra video mixing hardware. I assume the same is probably true for most video titlers. There's even a serial port on the back labelled "X-CON".

Here's a basic spec sheet I put together based on looking at the ICs inside:
CPU: Intel 80C31 microcontroller (Essentially an 8051 without the embedded ROM). Connected to a 6MHz crystal.
ROM: 32KB (27C256) for system firmware, 128KB (27C010) for character set
RAM: 32KB (62256)

If someone was willing to spend some time reverse-engineering one of these systems, they could theoretically program a proper operating system for it. Perhaps a BASIC interpreter or something similar? It'd certainly make for a unique project.

 

[GlueLogic]

It seems that a BASIC interpreter for the 8031 already exists, but there's no source code for it - just documentation on the changes that were made from the original Intel 8051 BASIC it's derived from. The 8051 BASIC does have source code, however.

 

[MiaM]

It seems that a BASIC interpreter for the 8031 already exists, but there's no source code for it - just documentation on the changes that were made from the original Intel 8051 BASIC it's derived from. The 8051 BASIC does have source code, however.

Well 8051 and 8031 are the same chip except that the 8051 has built in ROM while the 8031 always needs an external ROM (and thus also uses up a bunch of pins for connecting the ROM; on the 8051 way more pins can be used for general I/O). So those changes probably just changes I/O pin usage?

 

[paleotechnica]

The biggest challenge here is going to be reversing the video registers, more than anything. Is the video generator a bunch of jellybean logic chips, GALs/PALs, or custom ICs? It's probably worth taking an inventory of what you can find on there.

 

[GlueLogic]

Is the video generator a bunch of jellybean logic chips, GALs/PALs, or custom ICs? It's probably worth taking an inventory of what you can find on there.

The video circuit (excluding the analogue video mixing section) appears to consist entirely of standard 74HCxx logic.
Here's some photos. There are three PCBs in the system, they connect together via pin headers.

Digital Board (This is definitely where the graphics are generated, the EPROM on this board stores the character set):



Analogue Circuit:


Keyboard:

 

[syzygy]

I was reading through this thread and although I have no real interest in making videos or even titling them, it is an interesting morning coffee read so thank you.

I came across this project in progress (which you may have already seen). I am curious about the nomenclature, "circuit bending" in contrast to circuit hacking. Your potential project, adding a BASIC software would be circuit bending towards an enhancement (see video in the thread)?

Maybe I had too much coffee

 

[MiaM]

Well, it's "only" 32kbyte of code.

Maybe just try fiddling around with the code, like try to change some sign-on message or so, to see if it would get sad due to some checksum failure or if it just accepts any changes?

I assume that as you read the ROMs you also have an eprom programmer. Maybe get a few 28xx256 EEPROMs to make it easier to try different changes of the firmware, and maybe also insert a ZIF socket into the firmware socket so you don't need to risk damaging chip pins?

A ROM emulator would be the ideal thing though.

Btw to get some response from semi-helpful slackers like me I would suggest you start trying to disassemble the ROM and post snippets of it where it's unclear what it does or whatnot. I.E. I think I'm not the only one who wouldn't download and try to disassemble the ROMs, but might provide something useful if snippets are already pasted into the thread?