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RS-232 Driver Power Control And On/Off Output Clamp Circuits

IP.com Disclosure Number: IPCOM000100601D
Original Publication Date: 1990-May-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 95K

Publishing Venue

IBM

Related People

Gatson, MS: AUTHOR [+3]

Abstract

This article describes circuit arrangements for use in a personal computer for driver power control and on/off output clamping.

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

RS-232 Driver Power Control And On/Off Output Clamp Circuits

       This article describes circuit arrangements for use in a
personal computer for driver power control and on/off output
clamping.

      The line control circuits disclosed herein solve problems in
implementing the RS-232-C standard interface in a personal computer.
The problems occur on power down of the personal computer and are
caused by the inconsistency in which the two supplied voltages,  V
and 12 V, degrade to zero.  The  V degrades at a much faster
rate than  V.

      The standard RS-232 driver is powered by 12 V, but its
inputs are a standard transistor-to-transistor logic (TTL) interface,
driven by  V as illustrated in Fig. 1. Prior to power-down, these
inputs would be held inactive by a logic '1' or high TTL voltage
level, and the outputs of the driver powered by 12 V.  Upon
powering down the computer, however,  V begins to degrade at a much
faster rate than 12 V.  The inputs to the driver, therefore, have
time to fall from an inactive logic '1' state to an active logic '0'
state (or low TTL level) before power is removed from the drivers.
This logic switch results in pulsing active some, or more likely all,
of the RS-232 control and data lines transmitted by the personal
computer for a period from 20-30 ms to as long as several seconds.
This is a universal problem, because all known personal computer
adapters as well as some planars use some version of this standard
driver, and mostly all power supplies exhibit the same power-down
characteristics.

      The driver power control circuit of this disclosure shown in
Fig.  2 solves the problem by monitoring the volt power supply
voltage and upon detection of a power-off condition drives the output
of the RS-232 driver circuits to ground level, eliminating any d...