The Hewlett-Packard 4957A serial protocol analyzer is a tremendously useful instrument for debugging serial communication links. It is able to observe and display communications in both directions, even full-duplex, and includes decoders for protocols such as X.25, HDLC, SDLC, Bisync, etc. and character sets including ASCII and IBM EBCDIC. It includes a keyboard which allows injecting messages into the serial data stream.
Unfortunately, when it was released in 1992, Hewlett-Packard was completing its transformation from the engineer’s dream vendor to one which adopted the philosophy, “What the heck—let’s be evil”, and stopped supplying their products with schematics and documentation allowing them to be repaired by customers. Further, they “improved” the design by replacing industry standard integrated circuits with custom gate arrays whose programming was not documented. Three decades later, about the only way to repair these products of the “bad H-P” is to find one or more “parts machines” of the same model and swap components in the hope of getting one to work.