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hellothaimassage(at)yahoo
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 Posted: Thu Aug 09, 2007 9:35 pm Post subject: P-mag to VM-1000C tach signal |
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I've been trying to get my Vision Microsystems VM-1000C to read the RPM pulses coming from my P-mag ignition. After much testing, here is where I am:
Testing the ignition separately with an oscilloscope determined that it is putting out a 5V pulse with a 4ms duration. The bottom of the pulse (baseline) is at ground. Some of the pulses are slightly distorted on the back end, but I don't think that should matter.
Testing the VM-1000C separately with the scope and a frequency generator determined that it correctly counts pulses as long as the bottom of the pulse (baseline) is within 300mv of ground. If the bottom of the pulse gets higher than this, the VM-1000C doesn't see it.
However... When I connect the ignition to the VM-1000C and watch both of them together with the scope, I see that the VM-1000C raises the baseline of the ignition output from 0V to 2.7V. This is obviously way too high for it to count, and the RPM indicator thus reads zero.
The ignition people suggested putting a 0.47 microfarad capacitor in series between the ignition and the VM-1000C in an attempt to move the bottom of the pulse closer to ground. However, the capacitor actually raised the bottom of the pulse from 2.7V to 4.8V. That obviously doesn't help.
Can anyone offer me any guidance here? Something in the VM-1000C appears to be raising the baseline of the tach signal and preventing the pulses from being counted because they never get close enough to ground.
Please reference these images from the scope:
http://home.comcast.net/~n70ge/end_images/Ign.jpg shows the ignition output by itself.
http://home.comcast.net/~n70ge/end_images/ignvms.jpg shows the ignition output when connected to the VM-1000C.
http://home.comcast.net/~n70ge/end_images/ignvmscap.jpg shows the ignition output connected to the VM-1000C through the capacitor.
There must be a simple fix for this...
Thanks,
-Geoff
RV-8 ready to fly for the first time once I get this bug fixed
Ready for /body> [quote][b]
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bobf(at)feldtman.com
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| Posted: Fri Aug 10, 2007 4:38 am Post subject: P-mag to VM-1000C tach signal |
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Makes me think you need a better ground between the mag and the VM unit - sounds like a floating ground issue. Capacitor won't work, dodn't know why they suggested that. You could build a separate transistor based trigger circuit, but you shouldn't have to. Check your ground carefully and don't rely on the air frame. I'd use a separate large copper wire between the two
bobf
On 8/10/07, Geoff Evans <hellothaimassage(at)yahoo.com (hellothaimassage(at)yahoo.com)> wrote:[quote] I've been trying to get my Vision Microsystems VM-1000C to read the RPM pulses coming from my P-mag ignition. After much testing, here is where I am:
Testing the ignition separately with an oscilloscope determined that it is putting out a 5V pulse with a 4ms duration. The bottom of the pulse (baseline) is at ground. Some of the pulses are slightly distorted on the back end, but I don't think that should matter.
Testing the VM-1000C separately with the scope and a frequency generator determined that it correctly counts pulses as long as the bottom of the pulse (baseline) is within 300mv of ground. If the bottom of the pulse gets higher than this, the VM-1000C doesn't see it.
However... When I connect the ignition to the VM-1000C and watch both of them together with the scope, I see that the VM-1000C raises the baseline of the ignition output from 0V to 2.7V. This is obviously way too high for it to count, and the RPM indicator thus reads zero.
The ignition people suggested putting a 0.47 microfarad capacitor in series between the ignition and the VM-1000C in an attempt to move the bottom of the pulse closer to ground. However, the capacitor actually raised the bottom of the pulse from 2.7V to 4.8V. That obviously doesn't help.
Can anyone offer me any guidance here? Something in the VM-1000C appears to be raising the baseline of the tach signal and preventing the pulses from being counted because they never get close enough to ground.
Please reference these images from the scope:
http://home.comcast.net/~n70ge/end_images/Ign.jpg shows the ignition output by itself.
http://home.comcast.net/~n70ge/end_images/ignvms.jpg shows the ignition output when connected to the VM-1000C.
http://home.comcast.net/~n70ge/end_images/ignvmscap.jpg shows the ignition output connected to the VM-1000C through the capacitor.
There must be a simple fix for this...
Thanks,
-Geoff
RV-8 ready to fly for the first time once I get this bug fixed
[b]
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Eric M. Jones
Joined: 10 Jan 2006
Posts: 565
Location: Massachusetts
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| Posted: Fri Aug 10, 2007 5:40 am Post subject: Re: P-mag to VM-1000C tach signal |
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I don't think the 0.47 uF cap is right either. There may be a simple way to fix this thing. The P-mag people should know.
But failing all that, I have just finished a little circuit to convert 0-14.2V to 0.20 to 1.35 V to feed the R.A.C. bar graph indicator. The circuit uses one LM324 quad op amp. One of the op amps is configured as a non-inverting summing amplifier where Vout is the sum of V1+V2-V3-V4. This makes it simple to manipulate the output with small voltage dividers using the other op amps as buffers. PCBs are available. Email me offline.
Additionally, for those with a tacho frequency problem, I have designed a tiny device that converts 3 ppr to 1.5 ppr and maintains pulse width. PCBs are available. Email me offline.
"Everything you've learned in school as "obvious" becomes
less and less obvious as you begin to study the universe.
For example, there are no solids in the universe. There's
not even a suggestion of a solid. There are no absolute con-
tinuums. There are no surfaces. There are no straight lines."
- R. Buckminster Fuller
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_________________
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge, MA 01550
(508) 764-2072
emjones(at)charter.net |
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nuckollsr(at)cox.net
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| Posted: Fri Aug 10, 2007 7:02 am Post subject: P-mag to VM-1000C tach signal |
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At 10:34 PM 8/9/2007 -0700, you wrote:
| Quote: | I've been trying to get my Vision Microsystems VM-1000C to read the RPM
pulses coming from my P-mag ignition. After much testing, here is where I am:
Testing the ignition separately with an oscilloscope determined that it is
putting out a 5V pulse with a 4ms duration. The bottom of the pulse
(baseline) is at ground. Some of the pulses are slightly distorted on the
back end, but I don't think that should matter.
Testing the VM-1000C separately with the scope and a frequency generator
determined that it correctly counts pulses as long as the bottom of the
pulse (baseline) is within 300mv of ground. If the bottom of the pulse
gets higher than this, the VM-1000C doesn't see it.
However... When I connect the ignition to the VM-1000C and watch both of
them together with the scope, I see that the VM-1000C raises the baseline
of the ignition output from 0V to 2.7V. This is obviously way too high for
it to count, and the RPM indicator thus reads zero.
The ignition people suggested putting a 0.47 microfarad capacitor in
series between the ignition and the VM-1000C in an attempt to move the
bottom of the pulse closer to ground. However, the capacitor actually
raised the bottom of the pulse from 2.7V to 4.8V. That obviously doesn't help.
Can anyone offer me any guidance here? Something in the VM-1000C appears
to be raising the baseline of the tach signal and preventing the pulses
from being counted because they never get close enough to ground.
Please reference these images from the scope:
<http://home.comcast.net/~n70ge/end_images/Ign.jpg>http://home.comcast.net/~n70ge/end_images/Ign.jpg
shows the ignition output by itself.
<http://home.comcast.net/~n70ge/end_images/ignvms.jpg>http://home.comcast.net/~n70ge/end_images/ignvms.jpg
shows the ignition output when connected to the VM-1000C.
<http://home.comcast.net/~n70ge/end_images/ignvmscap.jpg>http://home.comcast.net/~n70ge/end_images/ignvmscap.jpg
shows the ignition output connected to the VM-1000C through the capacitor.
There must be a simple fix for this...
|
Without knowing more about the input-output characteristics
of the two systems, I'll have to wag it a bit. Sounds like
a classic dc-restoration problem reminiscent of the TV
sync separator designs many moons ago. Take a peek at:
http://www.aeroelectric.com/Pictures/Schematics/p-Mag_VM-1000_Interface.JPG
I'm fairly certain that the first circuit will work but is more
complex in that it requires power. The second circuit may work
and I'd try that first. If this doesn't get you running,
I'll contact P-mag and VM to see if we can take a more
predictable approach knowing the details of their input
and output circuits.
Bob . . .
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hellothaimassage(at)yahoo
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| Posted: Fri Aug 10, 2007 6:48 pm Post subject: P-mag to VM-1000C tach signal |
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Bob wrote:
Without knowing more about the input-output characteristics
of the two systems, I'll have to wag it a bit. Sounds like
a classic dc-restoration problem reminiscent of the TV
sync separator designs many moons ago. Take a peek at:
http://www.aeroelectric.com/Pictures/Schematics/p-Mag_VM-1000_Interface.JPG
I'm fairly certain that the first circuit will work but is more
complex in that it requires power. The second circuit may work
and I'd try that first. If this doesn't get you running,
I'll contact P-mag and VM to see if we can take a more
predictable approach knowing the details of their input
and output circuits.
Bob . . .
==========
After explaining all of this to Tom from E-mag, he (correctly) figured out that the tach pulse input line on the VM-1000C is biased hi (+5V). According to Tom, I have an older (1.5 years) version of the P-mag circuit board that uses a resistor to drive the baseline of the pulse to ground, and it doesn't work when connected to an instrument that is biased hi.
Newer versions of the P-mag circuit boards use an opto-isolator instead of a resistor to drive baseline of the pulse to ground. This newer board also has a configurable option called "open collector mode" that, in conjunction with the opto-isolator, should make this whole setup work properly. At least that's what Tom thinks, anyway.
Later today I got an email from an engineer at Vision (JPI). He confirmed that the tach input line was biased hi, and he suggested this circuit to fix it if the ignition couldn't be made compatible: http://home.comcast.net/~n70ge/end_images/Electronic_Ignition_Interface.pdf.
This circuit is slightly different than the ones you drew, Bob, but I assume it has the same function. Is that correct? Alternatively, Tom from E-mag suggested using a TTL 7404 driver chip if the upgrade to the P-mag board doesn't work.
Thank for your help.. I can troubleshoot somewhat and throw around big electronic words, but I'm certainly no circuit designer...
Interestingly enough, the drawing I linked to above from JPI is titled "Electronic Ignition Interface to EDM900/930/950/VM1000/VM1000C". If all of those instruments are biased hi on the input line, I wonder why this problem hasn't come up before with other instrument and/or ignition combinations.
-Geoff
http://smallbusiness.yahoo.com/webhosting
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