RPM / Frequency using a Tachometer

Hello, I'm new to the forum and new to LabJack so please be gentle :-)

I'm trying to configure this optical speed sensor as a tachometer to measure the rpm of a rotating blade.

http://www.omniinstruments.co.uk/displacement-position-sensors/optical-a...

The signal is connected to the FIO0 port on my labjack T7-Pro, and in the Kipling dashboard I see the FIO0 input go from low to high when the sensor senses the blade above it.

I want to get the frequency of this. I am trying to script this in the Lua, following the guide here:

https://labjack.com/support/datasheets/t-series/digital-io/extended-feat...

My Lua code looks like this:

print("connect the tacho signal wire (Yellow) to FIO0")
print("connect the tacho red wire to VS")
print("connect the tacho green wire to GND")

local mbRead=MB.R
local mbWrite=MB.W

local freq_seconds = 0
local freq_hertz = 0

mbWrite(44900, 1, 1) -- DIO_EF_CLOCK0_ENABLE = 1
mbWrite(44000, 1, 0) -- DIO_EF_ENABLE = 0
mbWrite(44100, 1, 11) -- DIO_EF_INDEX = 3
mbWrite(44000, 1, 1) -- DIO_EF_ENABLE = 1

while true do
  freq_seconds = mbRead(3500, 3)
  freq_hertz = mbRead(3700, 3)
  print("frequency in seconds: ", freq_seconds)
end

It doesn't seem to work - I get 0 for both the frequency in seconds and hertz.

Can anyone suggest where I have gone wrong?

Thanks,

Peter