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Differential Driver for High Common Mode Voltage Bus (EIA-485 Interface Driver)

IP.com Disclosure Number: IPCOM000107728D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 4 page(s) / 106K

Publishing Venue

IBM

Related People

Flynn, DM: AUTHOR [+3]

Abstract

In establishing an interface between separate clusters of computing equipment it is frequently the case that these clusters will be referenced at separate and distinct electrical potentials. Due to the discrepancies which may exist in ground shifts and power supply configurations this potential difference may be of the order of several volts (i.e., five volts) or more. Under these conditions there is a particular electrical standard known as EIA-485 (EIA = Electronic Industries Association), which must be adhered to for proper operation of the interface.

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Differential Driver for High Common Mode Voltage Bus (EIA-485 Interface Driver)

       In establishing an interface between separate clusters of
computing equipment it is frequently the case that these clusters
will be referenced at separate and distinct electrical potentials.
Due to the discrepancies which may exist in ground shifts and power
supply configurations this potential difference may be of the order
of several volts (i.e., five volts) or more.  Under these conditions
there is a particular electrical standard known as EIA-485 (EIA =
Electronic Industries Association), which must be adhered to for
proper operation of the interface.

      Described herein is the design of a bus driver circuit which
will meet the requirements specified by the EIA-485 standard.  The
salient features of the design are as follows:

      The driver output voltage is differential in nature, that is,
the opposite phases of the output state are available simultaneously
at separate I/O locations.

      The driver differential output voltage has a minimum magnitude
of 1.5 V over the range of operation with a differential resistive
load of 54 ohms.

      In the event of a fault whereby the outputs of the driver in
its active state are shorted to either -70V or +12V, the outputs of
the driver do not source or sink more than 250 mA of current, and are
able to withstand this fault condition for an indefinite period of
time without sustaining damage.

      In additi...