IREM IPM Invader

Tech and Mods page

last updated December 9, 2013


This is a pretty unique Space Invaders hack, based around the 6502 instead of the 8080.
Also features a 'coffee break' between waves! This seems to have been quite well
known in Australia, but i dont remember hearing or seeing any over here in the UK.

I was sent a pcb recently that wouldn't sync up with its JAMMA adapter.. and i found out why.

Firstly, the pinout i found online was incorrect in a few areas, so i've copied
and modified it so it is now accurate. My changes are in yellow:


IREM IPM Invader Pinouts
(M-11S-1 PCB confirmed)
Solder Side
    Parts Side
44 43 GND
+5 Volts
42 41 +5 Volts
+5 Volts
40 39 +5 Volts
Coin 1
38 37 (hangs the game if grounded)
36 35 N\C
34 33 N\C
32 31 Video Blue
Video Green
30 29 Video Red
28 27 N\C
Speaker (+)
26 25 Speaker (-)
+12 Volts
24 23 +12 Volts
+12 Volts
22 21 +12 Volts
** H Sync (negative)
20 19 -
18 17 V Sync (positive) 62hz
P1 Start
16 15 P2 Start
P2 Fire
14 13 P1 Fire
P2 Right
12 11 P1 Right
P2 Left
10 9 P1 Left
8 7 N\C
-5 Volts
6 5 -5 Volts
4 3 GND
2 1 GND


So the game uses seperate syncs, the horizontal sync is negative, and the Vertical sync is positive.

These days you are likely to want to play this game in a JAMMA cabinet or other monitor with
more 'normal' sync, which means it needs to be composite negative sync.

So we need to find where these syncs come from, go get your pcb and look here :

Fortunately right where the Syncs originate on the pcb, there's a 7400 with some unused gates :

VSYNC = 62hz

If you've read my PONG colour conversion page, you'll know that 7400's
are basic TTL building blocks, so its easy to make the gate you need.

We need to invert the V SYNC signal (its currently positive) and join it with
the H SYNC, thereby giving us a composite negative sync signal suitable for JAMMA.

To do this, we can join the 7400 NAND gate in the following manner to turn
it into an inverter like this :

So we simply join the two NAND inputs to our source, to give us an inverted output.


STEP 1 : solder pin 3 of E6 to the exposed leg of R177:

(This joins the output of the NAND gate to the HSYNC and edge connector pin 20)


STEP 2 : solder a wire from pins 1+2 of E6 to the exposed leg of R180:

(this connects the positive V sync to the input of our inverter)

and you're all done! wasn't that simple!?


Your pcb will now output composite negative sync on pin 20 of the
game pcb edge connector (marked with ** in the pinout table above) and
can be wired to a JAMMA adapter, or indeed a 'regular' arcade monitor.


Which pot does what?
[write to $a000]
0 VR2 base shoot
1 VR1 base explode
2 VR3 invader hit
3 VR4 Life Extend (bonus)
4 VR5 fleet move low tone
6 VR7 fleet move high tone
7 VR9 volume
VR10 controls frequency
- VR8
volume for ALL SOUNDS





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These pages are (C) Andy Welburn 1996-2002. I cannot be held responsible if the information supplied herein results in a blown monitor/power supply/house fuse/mind. Oh yeah, have a nice day :)