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A CIRCUIT FOR MULTIPLEXING EIA-232 SERIAL PORTS

IP.com Disclosure Number: IPCOM000008922D
Original Publication Date: 1999-Jan-01
Included in the Prior Art Database: 2002-Jul-24
Document File: 3 page(s) / 111K

Publishing Venue

Motorola

Related People

Lewis Clay Dearman: AUTHOR [+2]

Abstract

This paper describes a circuit for multiplexing EIA-232 serial ports for the purpose of unobtrusive monitoring of the communications that take place between two pieces of equipment. This method pro- vides the ability for a host processor to receive mes- sages transmitted by both pieces of equipment as well as the ability to break the communications path between the two pieces of equipment and take an active part in the communications process.

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MO7VROLA Technical Developments

A CIRCUIT FOR MULTIPLEXING

EIA-232 SERIAL PORTS

by Lewis Clay Dearman and Jerry Russell

INTRODUCTION

  This paper describes a circuit for multiplexing EIA-232 serial ports for the purpose of unobtrusive monitoring of the communications that take place between two pieces of equipment. This method pro- vides the ability for a host processor to receive mes- sages transmitted by both pieces of equipment as well as the ability to break the communications path between the two pieces of equipment and take an active part in the communications process.

PROBLEM(B) TO BE SOLVED

  The problem that needed a solution was to mon- itor serial EIA-232 communication between two pieces of equipment without adding any delay into the communication path while still retaining the ability to control the communications between the two pieces of equipment. The two solutions that cur- rently exist allow either unobtrusive monitoring or control, but not both. The eavesdropping monitoring method connects the transmit lines of each piece of equipment to a Universal Asynchronous Receiver/Transmitter (UART) device, allowing the host processor to receive messages transmitted from both pieces of equipment, but does not allow the host processor to stop the communications. Refer to Figure 1 for a block diagram of the eavesdropping solution. The other alternative is to connect the transmit and receive lines from each piece of equip- ment to a UART and have the host processor receive, interpret, and retransmit every message between the pieces of equipment. The interpretive solution is shown in Figure 2. While this method provides an opportunity for the host processor to control the communications, it effectively cuts the

transmission baud rate in half. A solution was needed that could provide the best of...