Innovate Motorsports SSI-4 - Manual (Page 10)

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5. Measuring Frequencies, custom RPM, or speed
The SSI-4 has the capability to measure frequencies on any of the channels. It converts a
frequency signal (pulses per second) into a number between 0 and 1023 (or 0..5V) to be logged
directly by LogWorks. This is useful for measuring custom RPM ranges, signals from speed
sensors or the frequency of MAF sensors with frequency output (as opposed to voltage output
MAF sensors).
The range of frequencies that the SSI-4 can measure can be programmed by with LM
Programmer. The SSI-4 can be set to any frequency range between 0 and 30 Hz for the full
0..5V range to 0..15 kHz for the full 0..5V output (logging) range.
Also, LM Programmer has convenient conversions built in, so you don’t have to calculate the
resulting frequency ranges for speed sensing or RPM sensing yourself. See chapter 10.x for
details.
A frequency input signal must have an amplitude (voltage range of pulse) between 0.5V at the
low pulse point to minimum of 3V and maximum of 40 V at the high pulse range.
See chapter 10 for details on different kinds of Frequency, speed and RPM sensors.
NOTE:
The custom RPM feature will work only for even fire tach signals, not for tach signals that
vary their time between pulses during an engine cycle (odd fire engines). Use the input 1
RPM functions instead.
6. Measuring duty cycle
To measure the duty cycle of a signal in channel 2, the input signal must cover the same voltage
range as for a frequency signal. Duty cycle is defined as the ratio between the time a signal is
active and the total time of the active and inactive time. A signal can be either active high ( the
event, like injector open, happens when there is a high voltage) or active low (the event happes
when the measured signal is at ground or close to it).
Very often the injector duty cycle is to be measured by the SSI-4. A typical fuel injector is
connected to 12V on one side, while the other side is connected to ground when the ECU opens
the injector. Because the pulse is therefore active when the voltage on the pin is at ground,
negative duty cycle is measured.
So called peak-hold injectors (as opposed to saturated injectors), work differently. Their drive
signal first goes to ground for a high current opening pulse, then rises to 8-10 Volts for the hold
period. Because the SSI-4 sees everything above 2.5V as “high”, it will be able to see only the
peak period. On some peak-hold systems it is possible to connect the CHx+ input of the SSI-4 to
12V at the injector and connect the CHx- input to the injector signal to still measure correctly. If
the above method can be applied, it actually measures positive duty cycle.
But that is not always the case. The LogWorks 2.0 Manual shows an alternative method.