DH11 asynchronous serial line interface

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The DH11 asynchronous serial line interface is a UNIBUS device which supports up to 16 asynchronous serial lines; as a UNIBUS device, it was usable on the PDP-11 and VAX computers. Modular 'line conditioning' units allowed support of either 20mA or EIA RS-232 serial lines.

Although it first appeared very early on in the lifetime of the PDP-11, it was extremely powerful and flexible. The parameters for each line (separate input and output baud rates, character length, stop bits, and parity) could be set dynamically (unlike, for example. the DJ11, which set parameters for groups of lines using jumpers). Output used DMA (with each line having its own buffer pointer and count), and on input a 64-character FIFO buffer made over-runs unlikely.

Initial implementation

The original implementation took an entire double system unit. It came in 'earlier' and 'later' versions: in the 'early' versions the line conditioning units were installed in a separate rack-mounted backplane, whereas in the 'later' units line conditioning was performed by boards mounted in the main backplane, and a rack-mounted passive distribution panel provided connectors for individual lines.