Hi Forum, In my quest abusing my FT-991A, I found that I can actually have two simultaneous CAT ports (already documented in this thread), with the catch that besides the Enhanced COM port from USB bus, you will need to use the RS-232 DB-9 port. I've been using happily this scenario at home. However, due to COVID-19, I'm staying at the country QTH, and I brought the 991A but forgot (ehr, sort of, it was just too complicated to remove) to bring the serial cable + Serial-to-USB adapter. And dual CAT is a must to me. Well, that got me thinking - I have already studied that part of the equipment when trying to grok the usage of the "Standard COM port" of my 991A and the conclusion is that Yaesu just didn't wire the TX/RX leads (Blue and Red arrows). As they do with every other product in their line-up, including the flagship FT-DX101D. Pin it. Now... what if I wired the Standard COM port by jumping the CMOS-to-RS232 driver TX/RX feeding lines? See the below schematic (click to enlarge): Here, I'm tapping the CMOS lines coming from the CPU that enters/leaves the Q1052, Analog Digital's ADM3202 CMOS-to-TTL driver in CMOS levels (3.3V) and jumping to the unused "Standard" RX/TX pins in Q1008 CP2105 Serial-to-USB bridge. There are two lines, a blue (TXD) and yellow (RXD). Fortunately, Q1008 provides a test pad plus a required 100R resistor in the TX line. Easy peasy, just land the TX line in the TP1001 pad. See the below blue arrow: However, the RXD pin (yellow arrow) does not provide any landing pad, connection or whatsoever. To give it a yay and complicate further things, Q1008 is a QFN chip. So in the previous schematic, I have added a 100R resistor (probably embedded on the wire) that would land on RXD data. This circuitry is located on the flip side of the Main Unit. But they are really close, and shouldn't be an hassle (fortunately there's a great technician in São Paulo): I don't intend to be working simultaneously with port DB-9 and USB; I will use USB permanently. So, do you envision any potential problem on carrying such mod? My only "concern" (notice the quote) is sharing the same TTL line between the Q1052 and 1008. And I'm sort of unsure/unconvinced why would this be a problem. ESD, shielding etc shouldn't be a problem, since this is all dealt internally in the radio; actually reducing the attack/problem surface by not using the DB-9 connector. Do you see other problems that I'm failing to see / perceive / think of? I'd appreciate a lot to learn about. 73, stay safe; - RF.