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Browse Prior Art Database

Differential Open Cable Detection Circuit

IP.com Disclosure Number: IPCOM000101182D
Original Publication Date: 1990-Jul-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 4 page(s) / 118K

Publishing Venue

IBM

Related People

Chen, SC: AUTHOR [+3]

Abstract

Disclosed is a circuit for improving the detection of faults in cables which connect two circuit cards and are driven by open collector drivers. The faults that can cause a detectable cable error include: missing or incorrect termination, an open cable, noise coupling onto the cable or in the case of bidirectional transmission lines, when both drivers are turned on at the same time on both ends of the same cable.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Differential Open Cable Detection Circuit

       Disclosed is a circuit for improving the detection of
faults in cables which connect two circuit cards and are driven by
open collector drivers.  The faults that can cause a detectable cable
error include:  missing or incorrect termination, an open cable,
noise coupling onto the cable or in the case of bidirectional
transmission lines, when both drivers are turned on at the same time
on both ends of the same cable.

      The problems with the single-ended open cable error detection
scheme are that it has poor noise immunity and minor tracking to
input changes.  The most frequent related problem encountered in
magnetic storage products is to post a false cable error.  When a
cable error is posted in read mode, it will initiate a retry, and
therefore, will increase the access time to obtain data.  If a retry
does not eliminate the cable error or if a cable error is posted
during a write operation, it becomes a hard error and the operation
will be aborted and a call for machine service will customarily be
made.  Therefore, a false cable error will degrade the performance of
a machine or cause a machine to remain idle until serviced.

      The foregoing and other problems of the single-ended open cable
detection circuit can be overcome by the implementation of the
disclosed circuit.  The noise immunity is doubled and the tracking is
improved.

      Fig. 1 shows the cable connections between an open collector
driver and a distant receiver including an open cable detection
circuit.  Assume the resistance of the collector pull-up is R and the
termination between the lines at the receiving end is 2R, then the
single-ended signal swing will be between -IR/4 and -3IR/4.
Therefore, the single-ended peak-to-peak amplitude is IR/2.  When the
cable opens, the single-ended signal swings between ground and -IR
and the peak-to- peak amplitude, in this case, is equal to IR.  The
single-ended open cable detection circuit takes advantage of the
differences in the signal levels and sets a reference voltage
midpoint between ground and -IR/4, at -IR/8 or between -3IR/4 and
-IR, at -7IR/8 as the threshold for dete...