Tachometer Square Wave Spec

[RexRoentgen]

I'm looking to get some help understanding the tachometer out specifications for an MSD 6AL ignition control unit. It specifies that "The Tach Output wire produces a 12 voltage square wave signal with a 22.5° duty cycle."

I'm fine with the fact that its a 12 volt square wave, but I'm a little lost on the 22.5° duty cycle. I think it is referring to a duty cycle that spans 22.5 degrees of one revolution of the distributor, but want to make sure 22.5° isn't a different way of specifying something like 22.5% duty cycle? If so, then can some one provide insight as to what they expect the base frequency to be?

I've got an email into MSD as well for their help, but as its the weekend I'd like to get an answer sooner than Monday.

 

[meBigGuy]

I would expect 22.5% of the pulse period to be high.

 

[RexRoentgen]

If that's the case though then how many pulses should I expect per revolution of the crank/distributor?

 

[Averagesupernova]

Being an owner of an MSD 6 I can tell you that just about any decent tach will read it. It is a standard tach signal. You don't say how many cylinders nor whether it is a 4 or 2 cycle engine so I cannot tell you how many pulses per rev of the crank. I don't recall reading the 22.5 bit although it has been a while. It is interesting to note that the number 22.5 is 1/16th of 360. So, if they refer to an 8 cylinder engine, with a 50% duty cycle per ignition event, then the square wave would be high for 22.5 degrees and low for 22.5 degrees repeatedly.

 

[RexRoentgen]

The thing is I am going be reading the signal into an MCU not a tach. Its an 8 cyclinder 4 stroke small block chevy.

The 22.5 degrees would then make sense for an 8 cylinder square wave with a 50% duty cycle based on what youre saying.

 

[meBigGuy]

The controller contains a processor and is programmed to the number of cylinders (factory set to 8). It reads like the 22.5% duty cycle is 1 pulse for every full cycle (for example, every time cyl1 fires). The docs also read as if it would be 22.5% regardless of number of cyl. So a 4 cyl, 6 cyl, and 8 cyl would all create 1 22.5% pulse per firing of the programmed number of cylinders.

 

[RexRoentgen]

Hmmm both suggestions seem reasonable. When I get my hands on the car and the ignition installed Ill hook up a scope to it to actually view the signal. I was hoping to have some code already laid down but i guess Ill just put a scale factor into account for pulses per rev.

Ill report back what I find out.

 

[Averagesupernova]

The MSD 6 series ignition modules have to take a trigger signal in order to output to the ignition coil. So if you are hooking it to a V8 your trigger signal will be 4 pulses per rev of the crank and so will the tach output. The only reason it is even provided is because the MSD modules are a capacitive discharge ignition with multiple discharges. This would be unsuitable for a tach to read directly off of the coil.
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Edit: I just skimmed through the on-line instructions. It refers to a 20 PERCENT duty cycle. Nowhere do I find it is referring to anything in degrees. The only reason there is a cylinder selection on the 6AL is so the rev limiter knows when to kick in. It has nothing to do with the tach.
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What are you triggering on? Inductive pickup in the distributor?

 

[meBigGuy]

6AL says 22.5 degrees http://www.msdignition.com/WorkArea/DownloadAsset.aspx?id=15032386116
6AL-2 says 25 degrees http://www.msdignition.com/WorkArea/DownloadAsset.aspx?id=15032386168
Another 6 Series doc says 20% (not degrees) http://www.msdignition.com/uploadedFiles/MSDIgnitioncom/Products/Ignitions/6420_instructions.pdf

The only reason I assumed 1 pulse per rpm is because they refer to it as an rpm signal, not a firing signal, and they have a micro-controller. That's all I was basing this on. I could easily be wrong. Especially since I thought tachs generally used a firing signal (4 pulses per rev for a V8).

 

[Averagesupernova]

Generally it is the tach itself that needs to know how many cylinders or how many pulses per revolution an engine makes. Most aftermarket tach's have a selectable number of cylinders or pulses per rev. Having worked for an aftermarket electronics/gauge manufacturer I know these things.

 

[RexRoentgen]

Sooooo is it 1 pulse per rev of the distributor or 8 pulse per rev of the distributor?

 

[Averagesupernova]

Sooooo is it 1 pulse per rev of the distributor or 8 pulse per rev of the distributor?

Any engine with a distributor will output however many pulses per revolution of the distributor that engine has cylinders. Every cylinder has to fire in one rev of the distributor. Sooooo, what's the problem?

 

[HowlerMonkey]

Do you even need the msd unit?

Can't you just output the waveform from the HEI distributor?

 

[Averagesupernova]

I don't think the objective of the MSD unit is to obtain a tach output. MSD creates a much hotter spark than conventional ignition systems.

 

[HowlerMonkey]

The stock GM HEI distributor has a pin in the connector that is labeled "tach".

 

[RexRoentgen]

The stock GM HEI distributor has a pin in the connector that is labeled "tach".

To my knowledge the stock 73 corvette does not have a HEI distributor and it also has a mechanical tach drive. I've converted the system over to an MSD ignition for reasons other than getting an electric tach signal out, but since I have it I'll use it for the project I'm currently working on.