The price reflected that they could also talk to mainframes, which didn't use an async bit stream. Couriers could support synchronous communications, and programs like CLEO could use that to support bisync and SNA protocols using the EBCDIC character set.

Instead of start bit, stop bit, and parity, it would use a few SYN bytes (00110010) to get the other side's clock in sync, and then send bytes back-to-back with no start or stop bits, then a checksum at the end of the block.

Back in the day, you had to talk bisync to sell goods at Walmart, for example. They just had a mainframe with Sterling Software's SUPERTRACS mailbox program, and a bank of 9600 baud synchronous modems. Basically in the 90s you had to learn some really arcane shit just to sell your whats-its there.

Thanks!