It's been an interesting morning. I've been digging around in the internals of the Siena SSD drive and made some discoveries.
The Siena SSD drive was released so that the diminutive Siena could still read SSDs. I bought one of these on eBay a week ago because I wanted to answer a question: "How did Psion get the SIBO Serial Protocol to work over RS-232?"
At first I thought there were two ways that Psion could have done this. First, the SIBO-SP packets (12 bits in length, although only 9 are useful) are repackaged so they fit into 8 bits. Or second, the Siena SSD drive tells the SIBO machine that it's SIBO-SP compatible and then changes from RS-232 using to SIBO-SP (maybe still using RS-232 signal levels that are downconverted for ASIC4/5?).
Turns out there was a third way.
The only documentation I have for the Honda connector is from a Series 5 service manual. In it, pins 10,13 and 14 are shown as not connected. Turns out that's not the case with the Honda SIBO machines (3c, 3mx and Siena). On the 3c board (pictured with the stripped-down drive) they connect to a MAX3212, an RS-232 transceiver.
This means that the Honda serial cable has a second set of serial connections, separate to the main RS-232 used for things like PsiWin.
The Siena drive has a passthrough RS-232 port with pins 9 to 14 disconnected. Having a second serial channel would explain how a Honda SIBO machine can do both regular RS-232 and read an SSD at the same time.
The white square in the top-middle of the Siena drive's board seems to be another custom ASIC! All the SSD pins connect to this. I've not seen any information about this chip anywhere. My guess is that it splits the single-wire SIBO-SP into two wires for transmission along the second serial link, but until I can get a logic analyser on it I can't say that for certain. The rest of the board must be providing things like Vpp for writing to Flash SSDs and lowering the 9v power supply to 5v.
This does sadly mean that, even with drivers, there is no way to get the drive to work on an EPOC32 machine, but it does potentially mean more exciting things for SIBO machines with Honda connectors, as it does look like there's a way to talk SIBO-SP over the Honda connector.