Many new laptops and PCs do not come with a serial port. These adaptors plug into a USB socket and allow you to connect up to your ECU.
Many cheaper adaptors give problems, these adaptors use a premium chipset and tested to work with Megajolt, Megasquirt and Emerald ECUs.
Individual coil per cylinder
2-input tacho driver
3-input tacho driver
4-input tacho driver
Please note that some rev counters can be particularly difficult and may require additional tweaks to work correctly.