Harry, I use this accessory -

http://www.oscium.com/products/mixed-signal-oscilloscope-imso-104

This is digital & analogue oscilloscope for iphone/ipad. I can easily pick up digital tachometer signal from obd1 on my 90 Porsche 964.

Oscium scope has 4 digital (0-7v) and one analog (-40v-40v ...