ELM627 Просмотр технического описания (PDF) - Elm Electronics

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ELM627

RS232 Break Detector

Elm Electronics 

ELM627

Example Application

Figure 1 shows the ELM627 installed in a remotely

connected, RS232 linked device. This is typical of many

connections used today - a 9600 baud serial connection

is made between a PC and a remote device. Serial data

is continually sent between the two devices, but control

of the remote system is by software only.

It is often quite desireable in such a case to be able

to reset the remote system to a known state under

software control. This can be either at startup, or if the

remote system fails to respond to signals for any reason.

The circuit shown below represents all that is

typically required to provide remote as well as power on

resetting capability. The ELM627 contiuously monitors

the transmit data line for a high level (3-25V) through the

100KΩ current limiting resistor. The second 100KΩ

resistor is only used to provide a pull-down to VSS,

should the data link become open circuited. Depending

on what other devices are conected, this resistor may

not be required.

Pins 6 and 7 are shown shorted to VSS, setting the

circuit to trigger on any high level that exceeds 5msec

in duration. This allows for triggering of the circuit on a

null character (‘00’) at 1200 baud, easily accomplished

with most systems.

The final connection to note is that the

pulse/continuous control (pin 4) has been tied to a high

level so that the generated reset pulse will always be of

100msec duration. This pulse width isn’t normally

necessary for the circuitry, but may be desireable if one

wants to connect an LED or a buzzer to the circuit for

feedback.

Several variations on this type of circuit are

possible. By noting that the ‘reset’ output can be used

for other control purposes, and possibly adding a toggle

type latch, the circuit can be expanded to remotely turn

on and off devices, leading to several automation

possibilities…

Host Computer

TxD

RxD

SG

9600 baud RS232 Link

Remote System

+5V

1

0.1µF

2

3

4

8

7

100KΩ

100KΩ

6

5

Figure 1. Remote Reset Control

to reset

circuitry

ELM627DSA

Elm Electronics – Circuits for the Hobbyist

< http://www.elmelectronics.com/ >

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