The step & direction circuits on the Gemini are designed as RS422 differential. Because the inputs are not optically coupled, they cannot accept 24V, except with the GC-SDA adapter kit (described below). As with any single-ended signal, it is much more susceptible to noise when it is NOT differential. Parker recommends using a differential driver circuit.
To use with a 5V single-ended indexer/controller, do not connect anything to the Step- input. Connect the 0V or GND side of the indexer to Digital Ground. Connect the Step signal of your indexer to the Step+ of the Gemini. Use the same procedure for the Direction signals.
If you measure across the Step+ and Step- with nothing connected, you will measure a 2.5V difference. This is because the plus input has a 681 ohm pull-up resistor to +5V, and the minus input has both a pull-up and pull-down resistor. This allows it to accept either differential or single-ended signals. When wiring it with a single-ended input, a 5V signal on Step+ will be 2.5V above Step-, thereby registering as "High;" likewise, a 0V signal on Step+ will then be 2.5V below Step-, and will be seen as "Low."
It is possible to drive the Gemini with 24V Step-and-Direction inputs, using the GC-SDA I/O adaptor. This is a small, external PCB with user-selectable resistors that attaches to the Gemini's 50-pin I/O connector. It adapts the Gemini to +24vdc, +12vdc or any other voltage level. This is listed on the website as an accessory option for the Gemini. http://www.parkermotion.com/manuals/gemini/GC-SDA_Guide.pdf