These I/O interfaces provide for communication over an asynchronous serial RS-232 or 20mA current loop link. Serial/parallel conversion is provided in hardware.

	12531B,C,D Interface Jumpers Jumper Function 12531B 12531C 12531D W1-A internal clock yes yes yes W1-B external clock yes yes yes W2 bit rate (BPS)   110 150 W3   220 300 W4   440 600 W5   880 1200 W6   1760 2400 U42-8     4800 U42-9     9600 U42-12     19200 W7-A transmit 2 stop bits   yes yes W7-B transmit 1 stop bit   yes yes

The primary functional distinction between the 3 versions is as follows:

• 12531B Buffered Teleprinter: RC oscillator for 110 BPS.
• 12531C Buffered Teleprinter: 225.28 KHz crystal oscillator for 5 rates from 110 to 1760 BPS.
• 12531D High-Speed Terminal: 307.2 KHz crystal oscillator for 5 rates from 150 to 2400 BPS.

The 12531A is not included here as it is a very different board.

The "High Speed Terminal" name for the 12531D is relative to the era: the provided jumper positions permit operation up to only 2400 BPS. It can function up to 19,200 BPS by jumpering to the other outputs of the clock divider IC (the U42-x entries in the table).

These boards are not double-buffered and there are some resultant timing constraints for software to deal with. Of particular note driver software may have difficulty dealing with reception of characters with only 1 stop bit. This is a consequence of the design of the board in that reception of a character is not completed or indicated to the program (setting of the channel flag) until the end of the first stop bit, at which point the following start bit has already begun. The design of the board is such that the flag bit must be clear at/near the beginning of the start bit for the board clock to synchronize properly on the character. To work successfully with 1 stop bit the program must clear the flag within a very short window after the flag has been set. With 2 stop bits the program has the entire period of the second stop bit to work with.