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Redundant termination of a communication line for maintaining the line properly terminated when one of the terminations is unpowered

Image Number 2 for United States Patent #4890271.

An input and output system has a plurality of tape drives (1-3) connected by a daisy-chained bus (5) to a processor (4). Each of two tape drives (2, 3) at the end of the bus has a terminator (30, 31) for transmit lines (6) of the bus. Each terminator of each line comprises a pull-down resistor (40) connecting the line to ground, and a pull-up resistor (41) and a diode (42) in series connecting the line to a source of potential (43). When a source is powered up, potential is present and a diode connected thereto is forward biased and supplies potential to the line. When the source is powered down and potential is withdrawn, a diode connected thereto becomes reverse-biased and prevents the powered-down source from grounding the line. Redundant termination of each line therefore allows any one tape drive to be powered down without precluding communications over the bus from continuing while the other terminator continues to supply the line with potential.

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