The personal computer detects the closing of the switch from a distance. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The personal computer detects the closing of the switch from a distance. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Note to the student

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Often systems that control personal computers need to detect the closing of remote switches or contacts at various points in time. Such short circuits can be detected by an IBM or compatible personal computer in two simple ways, each based on the application of a circuit and a BASIC program. In both cases, the serial port of a personal computer is used and no additional devices are required for the computer or changes in its circuitry.

In the first method, the program is executed continuously. Each time the line 130 command is executed, an attempt is made to send a print command to the communication port. The remote switch controls the "Reset to send" line of the port (Figure 1).

Rice. 1 (click to enlarge)

When the switch is open, the print command sent to the port is accepted for execution during the operation of the main program. When the switch is closed, when a print command is issued, the required "Reset to send" signal is not accepted, which causes a device connection timeout error.

Processing program:

100' Circuit breaker closed detection
105 ' Setting corn1: port. Setting the pulse duration "Reset transmission
"1 ms
110 OPEN "com1:9600,n.8,1,rs,cs1,cd,ds" AS #1,
115 ON ERROR GOTO 150 ' Set error trap 120 '
125' Main program
130 PRINT #1 ' An attempt was made to issue a print command to the communication port.
135 PRINT "Main program"
140 GOTO 125 ' reverse run in main program
145 '
150 ' Cycle with key closed
155 PRINT "Switch closed"
160 RESUME NEXT End of subroutine active with circuit breaker closed
With the switch open, the BASIC program uses
received signal "Reset to send". When the switch is closed
the BASIC program detects an error in the port operation when receiving
data in serial code and directs the execution of the program
to line 150.

The error handler (introduced on line 115) directs program execution to the subroutine beginning line 150, which is processed with the switch closed. This subroutine ends with the Resume next statement, transferring the program execution to line 135 to the execution of the main program. This loop will run every time the Print # 1 statement occurs in the program. When the switch is opened, command line 130 will be executed again, and the cycle associated with the closing mode will remain unclaimed.

Schematic and program improvements ensure accurate counting of circuit breaker closings. This uses a 555-series CMOS timer connected to the handshake line, generating a valid ASCII RS-232 character. Each time the switch closes, the circuit generates a 300-µs pulse (equivalent to one start bit and two data bits). The extra character generated is not essential to program execution.

The link buffer used by the BASIC program accumulates each character generated by the timer so that the number of key closures can be accurately counted. Each time line 225 is encountered during program execution, the buffer state is monitored. If any characters were received by him, they are read and counted by the subroutine beginning with line 245. The only condition is that line 225 must be executed often enough so that the buffer does not overflow. By default, the buffer size is set to 128 characters, but can be changed if necessary with the Open statement on line 210.

Fastidious users can transmit information during the duration of the one-shot pulse and control the reception of a special ASCII character. The duration of this pulse can range from the duration of one start bit (104 µs at 9600 baud) to the duration of the start bit plus eight data bits (937 µs). It is possible to use 9-level (rather than 9-bit) code. The duration of the pulse generated by the timer can be changed by selecting the resistor R3 and capacitor C3 or by changing the voltage at pin 5 of the timer. Although the program has proved its worth when working with BASIC and Quick Basic, sufficient opportunities for controlling the communication port provided by the described techniques can be obtained using other languages.

Literature

Vgise S. Trump. PC detects remote switch closures, ED, 1990, No. 13. pp. 73, 74

Author: Bruce Trump; Publication: N. Bolshakov, rf.atnn.ru

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