Last update - Mar 12, 2009

The DB15 connector needs to be a FEMALE connector (sockets as opposed to pins!) for the pin numbering to be correct.  I got excited and hasty, and stupidly installed an incorrect male connector on my first prototype board and blew the 12V trace clean off the board in one section, just like a fuse!  That was because the pin numbers end up being inverted if you use a male connector, so don't be stupid like me.  Make sure the board-mounted connector is in fact a DB15F FEMALE type!  Also, DB connectors are available with different amounts of setback from the mounting flange to the first row of pins.  For this board to work, you need a 0.318" short offset type connector.  Here's a list of parts from DigiKey that should work well, but you can probably find the stuff at a local electronics supplier quite easily as well, and for cheaper than DigiKey.

Board mounted connector: D-Sub, 15-Pos, 2-Row, FEMALE, 0.318" mounting, right angle thru hole, board locks, 4-40 nuts, DigiKey P/N A32120-ND
Jack screws: DigiKey 7230K-ND (you need two, or steal/borrow them from an old computer you've got lying around)
Mating connector: D-Sub 15-Pos, 2-Row, Male, solder cups, hollow unthreaded flange, DigiKey P/N A32503-ND or L717SDA15P-ND
Backshell Assembly: D-Sub, 15-Pos, 2-Row, Backshell, Metalized Plastic, Assembly Hardware, Strain Relief, Mating Screws 4-40, DigiKey P/N 915GME-ND

The diodes are optional, and only intended as a low average power flyback connection for switched inductive loads if someone is using a few of the board's transistor spots as general I/O with TIP120-122 transistors.  For me, the diodes were intended as a handy way of tying all possible flyback lines to one single pad, labeled '+12V" (and now that I think about it, I should change that label designator to something like FB to imply flyback.  Hmmm...)  Obviously the flyback needs to go somewhere, so you need to wire the pad labelled '+12V' to some 12V source, either in the MS box, or outside the MS box.  If you try to use the flyback diode on a PWM controlled output line, like a PWM idle solenoid or a boost solenoid, the flyback diode on that line may eventually succumb to being overpowered - I don't know, because I am still in my own prelimilary testing too.  If it does, we'll look at that when it happens.

A quick note about current limiting.  I don't really like it, but I included it on the board for some strange reason.  I don't like it because it forces the high current output devices to operate in linear mode, and that will create a LOT of heat that must be dissipated.  Whatever device they are controlling should be properly fused and the current limiting should be thought of as a last resort.  I only ever intended it for low current limiting, and not for ignition current limiting, although it might be able to handle it.  I will state for the record that I can't support that function, but if you try it on your own, keep an eye on component temperatures!  You don't want to set your car's carpet or dashboard on fire!  Always fuse things properly!  I am a pragmatic type, so I shy away from things like trying to get a transistor or fet to current limit 7 amps by running it in linear mode - it WILL create bags of heat, which is always the enemy.  It doesn't matter if it's a mosfet or a darlington bjt, they will both get smoking hot!

Q1 through Q6 are the active component of the current limiting system for each respective channel.  If you do use current limiting, the schematic says 2n3904, but you can also use a 2n2222 transistor or equivalent.  If you are using IGBTs for ignition, I'd leave the current limiting off, and set the dwell as per the standard methods listed in the MS documentation.  To bypass the current limiting, leave Q1-Q6 out, and bypass the current sense resistors (more info below) with a small piece of jumper wire (a clipped off lead from a 1n400_ diode would do fine).

R2, R4, R6, R8, R10, R12 are the current sense resistors. To determine an appropriate resistor value, 0.7 divided by your desired current will give you the nominal value for the current sense resistor.  The resistor's power dissipation is calculated as 0.49 divided by the resistor's nominal value.
Let's say you wanted to limit current to 1.5 amps.  0.7V / 1.5A = 0.466666666Ω.  You can't get such a resistor, so look at what's available in the desired range.  Here's a link to some nice resistors from Digi-Key -->

The 2W Panasonic resistors should fit the board just fine, and DigiKey P0.47W-2BK-ND would get you a 0.47Ω resistor, that would give you

0.7 / 0.47 = 1.49A limit, and 1.04W power dissipation at a sustained 1.5A limit.
If you don't plan to use the current limiting feature, you need to install jumpers in place of the current sense resistors.  I added extra pads at the one end of each resistor's placement to allow the jumpers to be a bit shorter - you can use long jumpers or short jumpers.  I used some 20awg tinned buss wire as jumper wire on my first test board.  As I said before, the current limiting was never intended for ignition current limiting, as that would require some higher power resistors, and honestly, the traces might not like that too much, assuming that ignition current limiting would happen around 7 amps or so.  Aside from that, for a 7 amp limit, you'd need a 0.1Ω resistor, which would need to dissipate 4.9 watts.  a resistor that big won't fit the board.  Because the average power in an electronically routed ignition system is low (25% duty cycle for a 4 cylinder, 17% duty cycle for a 6 cylinder), you might be able to fit a 3W resistor onto the board.  I would try a Digikey P/N 13FR100E-ND, but I make no guarantees about the board surviving a 7 amp current limit thrashing without some external fuse.  The resistor will have to dissipate 4.9 watts, and the transistor, or IGBT will be PUMPING out the heat at that point, and that resistor is only rated for a sustained 3 watts without even considering it's thermal derating factor.


In order to get any function from the board, you'll need to install 18awg magnet wire (or some equvalent copper wire with heat-shrink insulation over it) from:
OUT1 pad --> 1/2 connector pad
OUT2 pad --> 3/4 connector pad
OUT3 pad --> 5/6 connector pad
OUT4 pad --> 7/8 connector pad
OUT5 pad --> 11/12 connector pad
OUT6 pad --> 13/14 connector pad
The wires can be mounted to the back of the boards to keep things neat.  I know firsthand how this part kind of sucks, but no suitable alternative was possible at the time I designed the board.  Since then, I revised the circuit board as a more compact four layer board that does not require these jumpers; a panel of 4-layer prototypes will cost me upwards of $500, so I'm not too keen to order anything until I know there is a justifiable demand for those boards.


The pad labeled "15" connects to the DB connector's pin 15, and is unassigned.  You can either use "15" as a third ground pin by jumpering it to "9/10" (which is the power ground that should go back to the engine block), or you can use "15" as a place to bring the +12V pin out to the DB15 connector.