The PDP-8/E used two different serial interface cards, the M8650 which was build with standard TTL logic and the M8655 which used a UART (Universal asynchronous receiver transmitter) to handle the serial to parallel conversion.
The M8650 is configured by soldering jumpers across posts on the board. The board only officially supports baud rates up to 1200 or 2400 depending on board revision. The board can operate to very high baud rates if jumper wires are soldered to a chip pin. I ran one of mine at 38.4k with no problems for transferring drive images to my PC using my dump/restore programs.
M8650 configuration information
The M8655 is configured by switches and jumpers. The UART chip used can only operate to 9600 or 19200 depending on the particular UART chip on the board. This board is also capable of handling parity in hardware.
M8655 configuration information
These boards use a 40 pin dual row header (like used with ribbon cables) for the serial interface. These were wired with discreet wire with crimp contacts. The boards support both standard RS-232 or a current loop for teletypes.
M8650/M8655 (BC01V) serial cable information