Here is the schematic of the simple CAT interface I use with my FT-817.
It is very simple and takes power from the FT-817 itself, so you want to make sure it is unplugged when you run off batteries.
It does not meet the EIA-232 standard for the output voltage, but I have not found a computer made in the last 10 years that did not work well with it.
With this interface, the output voltage is 0 or about +13V, where EIA-232 requires that the voltage be either more negative than -3VDC, or more positive than +3VDC.
I hope you are as lucky as I am.
This interface works well in conjuction with my Dual Radio Interface described elsewhere.