1. Xcat pin 16 (blue) to Syntor X U6 pin 2 on the personality board. I tacked this directly to the IC chip as shown in photo 1.
2. Xcat pin 4 (yellow) to Syntor X J1-35. This is an unused pin in the Syntor X control cable. This brings out the serial data to connect to the "Doug Hall" data line on the repeater controller. See photo 2.
3. Xcat pin 18 (gray) to Syntor X J1-36. This also is an unused pin in the control cable. This brings out serial clock for "Doug Hall" control. See photo 2.
I soldered a fourth wire to test band and PL/DPL scanning. Scanning won't work in "Doug Hall" mode but it's fun to play with on the bench. Xcat pin 17 (violet) goes to U2 pin 15 on the personality board. You must make the COS input selection on the configure tab of the Windows control program. If you forget to do this the audio will be turned off or muted.
Be sure to go to the Xcat site and download the xcat_docs.pdf file. As always RTFM!
Control Cable
One could use a control head but it's overkill for a remote base. Besides, mine didn't come with one and they seem hard to come by. If you do use a control head it must be on mode 1. The only control really needed is the squelch. I also wired up a volume control and a speaker for testing. A number of pins need to be grounded or pulled to 12 volts as indicated to make the Syntor X function. Here's a schematic (pdf) of how I wired the cable.You'll need what's sometimes called a charlie-zero-one key to remove the pins from the cable. See repeater-builder.com keys page (look all the way down to the bottom of the Motorola section) and a picture of the key. The narrow part of the key goes towards the body of the pin in order to press on the release. The the pin body (and release) is opposite of the wire crimp. Push the key in hard from the front and pull the wire and pin out the back.
I scrounged up an old 800Mhz base station and stripped most of the parts out of it. I used linear taper pots for both the squelch and volume. That's fine for the squelch but I suspect the low volume adjustment would be less abrupt if you use an audio taper.
Audio
The audio gate board may not be needed if your repeater controller gates the audio. I'll post a detailed schematic of the audio gate after I draw it. It's just a 4066 and a 4011. You could do it with a 4053 but I used what I had. I used an on-board 5 volt regulator to keep the levels compatible with the Syntor X and my repeater controller. My Syntor X has a 1.3 volt DC bias and 1.6 volts AC on the DET OUT audio. I didn't have any problems with supply rails or popping to do DC switching.DET OUT is discriminator audio, it needs de-emphasis, so remember to adjust you controller appropriately. On the Pacific Research PE-3 you need to move the jumper from the default of pins 1-2 to pins 2-3 for whichever port you're using. The transmit audio jumper stays on the 1-2 default pins (pre-emphasis).