The 40-pin GPIO connector on the Pi is attached to the 50-pin SCSI connector through a few 74LS641 transceivers with a few resistor packs for pullups and pulldowns. The software allows for virtual ...

Of course, these days, you are more likely to find some version of SATA and — lately — NVME connectors. But a standard predating all of this was very common in high-end systems: SCSI.