The actual protocol is called 3WI even if only two wires are used because the data line is bi-directional. Most adapters don't use the sense line (which I believe is only actually used for KVL to KVL transfers at this point). But yes, the SEM outputs the data to RS-232 then uses a protocol level converter to convert to 3WI in the actual keyload cable.
The obsession with a Hirose adapter...funny because the 5300 standard control heads uses a Hirose connector for the microphone jack, the 5300ES wearing the Lightning heads uses a Hirose connector for the microphone (a different one) as do the Viking control heads (again, different) which you'll also find on first and second generation Viking mobiles. KNG mobiles also use a Hirose adapter for the microphone.
Anyway, I have nothing against using Hirose on the KFD side. I'm actually trying to modify a EFJ keyload cable to Hirose on the subscriber end to see if it will keyload Motorola subscribers right now because a VK-5000 is not only more attractive cost-wise than a KVL-5000 but also more available right now. That being said, here's my daily loadout for keyloading with a KVL-4000 at work:
- KVL to KVL cable (i.e. MX to MX) (~$82 from a online Motorola dealer)
- KVL to Hirose cable (~$286)
- APX Portable Keyload adapter (~$61)
- GCAI Keyload adapter (~$180)
- W-series Keyload adapter (~$40 on the bay NOS)
- XTS Keyload adapter (which has also been modified to keyload EFJ 5100's and VPx00's) (~$25 on the bay NOS)
- KVL to Kenwood Multipin (~$272)
- KVL to Kenwood 8P8C (~$252)
- KVL to Viking CH (~$275 is what I paid for it but I think the dealer let me have it close to cost)
- KVL to BKR (~$528)
Why'd I post that? Well...Hirose keyload adapters are significantly cheaper by at least $100 and they don't lock you into a specific vendor's KFD. One keyload cable to Hirose and I can keyload 7 different Motorola and EFJ series of radios that are still in use on my local system and two full generations of Motorola consoles. There is significant cost savings there. Unfortunately, the Hirose connector (which is $15 from Mouser) is honestly one of the cheapest and most versatile options.