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Coding Circuit for the Conversion of a Programmed Resistor into a 4 Bit Word

IP.com Disclosure Number: IPCOM000045643D
Original Publication Date: 1983-Apr-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 82K

Publishing Venue

IBM

Related People

Lalla, P: AUTHOR

Abstract

When a modem is installed by a customer to interface a Central Exchange, for example, a Computer Processing Unit (CPU), the emission level of the modem is adjusted in the Central Exchange with an appropriate resistor (Rx). It is often desired that the CPU sense the value of this resistor, and convert its analog value into a binary word, e.g., a 4-bit word.

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Coding Circuit for the Conversion of a Programmed Resistor into a 4 Bit Word

When a modem is installed by a customer to interface a Central Exchange, for example, a Computer Processing Unit (CPU), the emission level of the modem is adjusted in the Central Exchange with an appropriate resistor (Rx). It is often desired that the CPU sense the value of this resistor, and convert its analog value into a binary word, e.g., a 4-bit word.

This word is stored in a random-access memory, and the CPU may, at any time, use this word for various computer or control operations.

The resistor Rx is mounted in parallel with a resistor bridge (typically, 3.57 kOmega and 1.19 kOmega) and may take any value between 0 (short circuit) and Infinity (open circuit), such that the attenuation varies from 0 to 12 dB, by 1 dB jumps. The correspondence between the attenuation A (dB) and the Rx value is given in the table shown in Fig. 1. The final coded word will therefore represent a log function of Rx.

The coding circuit explained in detail hereafter is shown in Fig. 2. The circuit will be a part of the modem and will ensure the dual function of sensing the value of Rx and converting its analog value into a 4-bit word, as explained above.

Part I of the coding circuit is a linearization circuit; it ensures the conversion of discrete analog values of Rx, following a logarithmic law into a stepped linear voltage Vx, each of the 13 steps being represented by a change of about .290 volt. The resulting voltage Vx is shown in Fig. 1.

From the 13 steps shown in Fig. 1, two sets of voltage steps are generated (one of 7 steps and the other of 6 steps) according to the following technique.

A first comparison is effected in part II of the circuit with...