Data Acquisition System Based on LabVIEW

1 LabVIEW Partial Design

1.1 VISA Introduction

LabVIEW provides a powerful VISA library. VISA (Virtual Instrument Software Architecture) - Virtual Instrument Software Specification is a general term for standard I/O function libraries and related specifications used for instrument programming. The VISA library resides in the computer system, completes the connection between the computer and the instrument, and is used to realize program control of the instrument. Its essence is a standard API for virtual instrument systems. VISA itself does not have programming capabilities. It is a high-level API that realizes instrument programming by calling the underlying driver. Its hierarchy is shown in Figure 1. VISA is an I/O interface software that adopts the VPP standard. Its software structure consists of three parts, as shown in Figure 2.









Compared with other existing I/O interface software, VISA's I/O control function has the following characteristics: It is applicable to various instrument types (such as VXI instruments, GPIB instruments, RS-232 serial instruments, message-based devices, register devices, memory devices, etc.); It is applicable to various hardware interface types; It is applicable to single-processor and multi-processor structures or distributed network structures; It is applicable to a variety of network mechanisms.

The source code of VISA's I/O software library is unique. It has nothing to do with the operating system and programming language. It only provides a standard API file as the output of the system.

1.2 Serial communication functions in the VISA library

The main serial communication function call path used in this article is: Functions?Instrument I/O?VISA?VISA Advanced?Interface Specific?Serial.

(1) VISA Configure Serial Port node (as shown in Figure 3)

　



This node is mainly used to initialize the serial port. The main parameters are as follows:

VISA resource name: VISA resource name, which refers to the serial port number in this article. Baud rate: baud rate, the default is 9600.
Data bits: the number of bits in a frame of information, LabVIEW allows 5 to 8 bits of data, and the default value is 8 bits.
Stop bits: the number of stop bits in a frame of information, which can be 1 bit, 1.5 bits or 2 bits.
Parity: parity setting. It can be no parity, odd parity or even parity.
Flow control: The data type of this parameter is cluster, which is used for handshake mode in serial communication.

(2) VISA Read node (as shown in Figure 4)?

　



This node is the serial port reading subVI, which is the main node in this article. It reads the data in the serial port and then uses the powerful data processing function of LabVIEW to analyze and process it. The main parameters are as follows:

VISA resource name: Same as above.

Byte count: Used to set the number of characters to be read. Since the serial communication subVI of LabVIEW only allows the reading and writing of strings, the correct conversion between strings and numbers must be achieved when processing data in this article. In addition, if you want to read all the characters in the current serial port, you must execute the "VISA Bytes at Serial Port" subVI to determine the exact number of bytes to be read, and then use its output as the input of the VISA Read node. [page]

(3) VISA Close node (as shown in Figure 5)?

　







This node is used to close the open VISA resource. There is only one main parameter:

VISA resource name: Same as above.

The baud rate of the LabVIEW serial port communication program used in this article is 9600, no parity check, 8 data bits, 1 stop bit, and software and hardware handshakes are prohibited. 2.1 Hardware The

MCS -51 microcontroller has a powerful full-duplex serial port inside. The serial port has 4 working modes. The baud rate can be set by software and is generated by the timer/counter inside the chip. Both receiving and sending can trigger the interrupt system, which is very convenient to use. There are two physically independent receiving and sending buffers SBUF, and there are also two independent receiving and sending signal lines RXD (P3.0) and TXD (P3.1) to the outside. This article adopts the RS232 serial interface standard. In terms of electrical characteristics, RS?232 uses negative logic, requiring a large amplitude between the high and low signals. The standard is: logic "1" is between -5V and -15V, and logic "0" is between +5V and +15V. Usually, about -10V is used as logic 1 and about +10V is used as logic 0. Since the signal input and output of the MCS-51 system are TTL level, the logic 1 is about 3.8V, and the logic 0 is about 0.4V, therefore, an external circuit must be connected to realize the conversion from TTL level to RS-232 level. This article uses MAX232E to realize this conversion. The TLC0831 chip is used for data acquisition. The TLC0831 chip is an 8-bit successive approximation voltage type A/D converter, which supports single-channel input and serial port output, with fixed polarity settings and no addressing required. Its main features are: 8-bit resolution; 5V power supply provides an adjustable reference voltage of 0~5V; input and output are compatible with TTL and MOS. If you want to increase the sampling frequency of the signal, just choose an A/D chip with a faster conversion speed. The schematic diagram of the hardware part is shown in Figure 6. 2.2 Software part The main program of the microcontroller software part is given below. The baud rate is set to 9600, and the baud rate is generated by timer 1. The serial port works in mode 1 without parity check. Timer 0 sets the sampling time interval. TLC0831 is the A/D converter data sampling subroutine, and SEND is the microcontroller sending subroutine. 3 Conclusion This paper uses a microcontroller for front-end data acquisition, realizes data communication with LabVIEW through a serial port, and uses the powerful signal analysis and processing function of LabVIEW to develop a data acquisition and information analysis system with low investment and easy operation (its virtual panel is shown in Figure 7). Experiments have shown that the system works well.