Using 2716, 2732 on Space Invaders

joeyoravec

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I'm fixing a Taito Space Invaders TVN boardset. This post shares what I've learned about modifying the Taito and Midway boardsets to use other kinds of EPROMs. I don't own a Midway boardset so I've only verified the info for Taito.

My board came with 2708 EPROMs which really stinks. 2708s are ancient and difficult to buy. 2708s require three voltages (+5, -5, +12v) so my EPROM programmer cannot read or write them. Fortunately 2708, 2716, and 2732 eproms have a similar pinout (link). By modifying pin 19, and pin 21 it's possible to substitute a more common chip like 2716 or 2732.

Both Taito and Midway board have jumpers to configure those lines. Midway published a table (link, final page) that offers settings for 18 outdated chips, but not 2716 or 2732. Taito published nothing that I could find. Taito scans on xmission are low-resolution and unreadable so use this instead (link).

Here's an explanation of the two boardsets:

Code:
Midway    Taito    Purpose {settings}
S1        S1       ROM pin 22 {5v, A9, GND}
S2        S2       ROM pin 21 {-5v, A10, GND}
S3        S3       ROM pin 19 {12v, 5v}
S5        S4       ROM pin 18 {5v, GND}
S6a       S5       ROM select bit 0 {A9, A10, A11}
S6b       S6       ROM select bit 1 {A10, A11, A12}
S6c       S7       ROM select bit 2 {A11, A12, A13}
          S8       ROM select bit 3 {A12, A14, GND}

You'll notice some useful settings are missing. S3 doesn't offer A10. S2 doesn't offer +5v or A11. The jumper pad is used to get a signal into the chip.... but you might have to find the signals nearby and run a longer wire. Now you have a few options:

  • Use 2716 (original number): S3 Pin19=GND. S2 Pin21=+5v. Program 1k images to the bottom-half of each chip.
  • Use 2716 (fewer): S3 Pin19=A10. S2 Pin21=+5v. Change ROM select to {A11, A12, A13, GND} so each ROM socket has 2k. Combine images, program 2k to each chip, and install in the correct sockets.
  • Use 2732 (original number): S3 Pin19=GND. S2 Pin21=GND. Program 1k images to the bottom-quarter of each chip.
  • Use 2732 (fewer): S3 Pin19=A10. S2 Pin21=A11. Change ROM select to {A12, A13, GND, GND} so each ROM socket has 4k. Combine images, program 4k to each chip, and install in the correct sockets.

I'll probably install the original number of 2732's, since it's so easy to get ground to those pads. But any of those work. I'm not sure if Midway has a jumper for ROM select bit 3, so you might be limited there.

I've attached a photo of my Taito TVN board with the signals. Unfortunately KLOV forum shrank the image and it's extremely difficult to read, so probe it yourself with a DMM. Here's a guide that shows you what to do on the Midway boardsets (link).
 

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If you look at the schem...each rom is enabled by a decoder chip (look at the OE pins for each rom and see where they go). Once you get back to that decoder chip that should have all the address lines you need. Those addy lines to the decoder on the midway set is jumpable, I believe so that the decoder will work properly for larger roms. (because if its set up for 2708s...despite the socket pinouts, if it needs to get to an address past the first 8k it'll turn off the OE pin on the first socket and jump to the next.
 
Regarding Midway, I re-checked the schematic (link) and yes the decoder can be rejumpered for larger ROMs (2k / 4k) equally easily. Notice that Midway has fewer ROM sockets so they decode only 3 bits and ground the 4th (perfectly fine for this!). That's what threw me off.

I should point-out a small difference. Taito 3-board TVN adds an additional buffer on the ROM board, creating a buffered ROM Address RAx, which does not exist on the Midway version. So if you convert a Taito to the larger chips, I'd suggest using the buffered version of address RA10/RA11 from IC22. You're jumping almost 3 or 4 inches to the S3/S2 pads and the buffering might help signal integrity. Worth checking with an oscilloscope.
 
Ok, completed this conversion but I forgot to mention that you have to remove the decoupling capacitors (credit to this site for reminding me):

attachment.php


Note the blue and red bars. The board originally had decoupling capacitors on the signals leading to pin 19 (red) and pin 21 (blue). That's good for VEE/VDD but too much load for pin19=A10 or pin21=A11. So if you do that rework, make sure to remove those capacitors. If you don't it's going to look like the address line is stuck low.

Note: pinouts and DIP switches available on the internet were wrong for my TVN board. Consult the MAME driver (link) for the most accurate info. Most importantly notice with an "sitv" ROM set that SW3 controls test mode.

Note2: I flashed "sitv" which includes a test mode. That ROM only varies from "sicv" by a few bytes total, plus a 1k block for test mode. So I think "sitv" is the better option but check it out for yourself.

Note3: My trimline is wired with a single set of controls, but on the 2 player game the "sitv" ROM is checking the 2nd player's inputs as if it were a cocktail. There's a suggestion (here) for converting a board for cocktail / upright but this layout does not exist on a TVN board. I didn't see any cocktail / upright DIP so I'm jumpering some wires to make it work in the trimline.
 

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Last bit of puzzle; Thanks

Hey, great post! I'm wondering if you can illuminate something for me. I've learnt a lot while at the same time got confused. lol.

I'm fixing a Taito Space InvadersTVN boardset. This post shares what I've learned about modifying the Taito and Midway boardsets to use other kinds of EPROMs. I don't own a Midway boardset.....

[*]Use 2732 (original number): S3 Pin19=GND. S2 Pin21=GND. Program 1k images to the bottom-quarter of each chip.
.

Could you please confirm the (original number) of 2708's you speak of above. I'm guessing there were eight?

Thanks,
Ro
 
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