usbs have microchips that accept code updates
USB "converters" should be considered suspect. Plugging BadUSB's, BadHDD, CPU's, Flash, or any other chipped / smart device or port with firmware, microcode, chips etc between systems has potential to infect / attack them. Assuming some random magical usb converter cable sets do pass raw rs-232 between them (ie: can cut/splice to a rs-232 port / modem / teletype) users often probably fuck up and cross infect usb during the n-th insertion setup session. Various "air gap", all adaptable to 'cat hugefile > /device'... QR code OCR scanning Sound Light RF Keyboard bots Monitor display output to camera capture input, a digital stream of bits thrown onscreen as fast as the two can sync. Simple RS-232 protocols, ECC codes, etc. All assuming endpoint chipsets don't attack over the gap / wire. Keep simple enough to see, log, debug, verify, filter, audit... like ASCII. USB, optical disk, tape, hdd... often have media based firmware update mechanisms, exploits, special sectors, bootcode, emulation, etc.
scrabble tiles
As received from the store... exhibit a non-random character frequency count, should not be used without adjustment down to 1:1.