A number of Yaesu owners might have noticed that upon plugging the rig to your USB computer port you will be presented both an "Standard" and "Enhanced" COM ports. And if you read the manual properly, you will also notice that you should not use the "Standard", but use the "Enhanced" port for CAT, right? Very well. But what about the "Standard" port? What's the use? Well, I finally found - mostly by accident. Reading the FT-101D review I found this nugget: "The enhanced port can be used for CAT functionality, while the standard port is used for PTT connectivity" So I went to check the schematics and do some testing. Indeed, in the 991/A and in the 891, the "Standard" port has its use for PTT control. Allow me to elaborate. By checking the schematics of the FT-991/A and FT-891, I have found that the serial-to-USB chip in the radio indeed provides two independent serial devices, the Standard and Enhanced. At the Enhanced port, you will find wired the TX, RX (these used for the actual CAT command), CTS, RTS and DCD pins. However, you will also find the DTR and RTS pins wired to the Standard port. So, finally, the Standard Port is meant to PTT or RTTY Shift control. As far I have tested, I can't command a PTT event using DTR or RTS pins in Enhanced port - these must be done at the Standard port, with a gotcha: In FT-991/A, the menu "071 - Data PTT Select" using the desired pin that you want to want to act as a PTT: the DTR or RTS. If you want to use this pin to control the PTT event in other modes, the same must be done in other modes in menu 047 for AM, 076 for FM, 098 for RTTY and 110 for SSB. Of course it is simpler to just use the CAT command to command a PTT down/up event, but... Well, here it is, hope it can be of help for someone else. 73 de PY2RAF - Rodrigo.