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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Pairing a digital multimeter with a computer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology Connecting a small-sized multimeter to a personal computer allows statistical processing of the results of a series of measurements. For example, it is possible to investigate the spread of parameters of a group of components or changes in voltage and capacity of batteries during discharging. One can imagine a number of other applications of such a "tandem", the creation of which is described in this article. Recently, multimeters of the 830 series, for example, DT830 or M-830, have become widespread among radio amateurs. They have a relatively small error, which allows them to be used for a wide range of measurements. Using the proposed device, you can enter data from the multimeter into a computer for further processing. Multimeters with this feature usually have an RS232 interface and are relatively expensive. The proposed adapter is made on inexpensive widely available components. Numeric data is read directly from the multimeter's ADC pins and transmitted over a serial link. It is not recommended to use multimeters for this refinement, in which an ADC chip is installed in a frameless version. The heart of the 830 series multimeters is the ICL7106 ADC (domestic analogue of K572PV5; description can be found in [1]). The description of the operation and the scheme of the multimeter can be found in [2, 3]. The ADC interacts with the LCD through static control [4] - each image element is controlled through a separate output of the microcircuit, to which rectangular voltage pulses are applied, phase-shifted by 0° or 180° relative to the pulses applied to the common indicator wire. If the phases on the LCD outputs coincide, the segment is not excited. The proposed device consists of two parts: a block for converting data from the ADC (LCD multimeter) and a block for transmitting data to a computer. In the conversion unit, CMOS shift registers with parallel loading DD1-DD3 are used to determine the state of low-current control outputs of the indicator (Fig. 1).
The device works as follows. At a low level at pin 1 of registers DD1-DD3, asynchronous loading is performed. After a high level is applied to this pin (via the RD line), data is fixed, which is shifted along the edge of the clock pulses at pin 2. The data is taken from pin 9 of the DD3 register to the DATA bus. Since the seven-segment code is redundant (bits of these / - "superfluous"), information about commas can be additionally transmitted in these bits. This information is taken from terminals 12 and 16 of the LCD multimeter. These pins can be connected to the collectors of the transistors or directly to the multimeter's multi-position switch. This switch, in turn, switches them directly to the battery positive (high). This state does not allow distinguishing between commas at a high level on the BP pin (ADC pin 21). Both commas will be canceled, since there is a high level on the outputs 12 and 16 of the LCD. The data transfer unit can be built in various ways. Its simple version is shown in Fig. 2. It serves to match the LPT port and is completely housed in a suitable XS1 connector housing. Power is supplied from an external source with a voltage of 9 ... 15 V. XP2 and ХРЗ connectors are connected using a flat ribbon cable with corresponding mating connectors - IDC-10F. The XP2 plug may not be present if the cable is connected directly to the port. With the XP2 connector disconnected, the DD1-DD3 microcircuits are de-energized, and the multimeter can be used in the usual way. The data transmission is completely controlled by the computer. The source code of the control program for DOS is in the file m_lpt.cpp [5].
The given version of the block does not have galvanic isolation, so it should be used with great care. For example, a voltage of 30 V entering the LPT port during a breakdown in the ADC chip can damage the motherboard. To eliminate this shortcoming, a more complex scheme of the data transmission unit was developed (Fig. 3). It is a microcontroller unit with galvanic isolation and transmitting data via a serial RS232 channel. The use of a single-chip microcontroller made it possible to minimize power consumption and reduce dimensions.
The PIC12F629 microcontroller has 1024 FLASH words of program memory, 64 bytes of data memory, 6 I/O ports, and an internal 4 MHz clock. It does not have a hardware transceiver (USART), so the RS232 protocol is reproduced by software. The microcontroller is powered by an internal 4 MHz clock generator, for which software calibration is provided. Also, the block can use the PIC12F675 microcontroller, identical to PIC12F629 with an additional four-channel ADC (10 bits). Other parameters of these microcontrollers and technical documentation can be found in [6, 7]. Programming can be done using the EPIC programmer. The firmware is shown in the table. All elements of the block according to the scheme of Fig. 3, with the exception of the XP4 connector, can be placed inside the multimeter case, connected to the COM port with a conventional modem cable. Information data is issued in two-byte packets upon request. A request through the U3 optocoupler is formed at pin 7 DD5 by a signal drop from high to low, which corresponds to the transmission of a zero byte by the computer. After receiving the request, within 3 ms, the data is loaded from the registers DD1-DD3 and converted. Next, the first byte is transmitted (2 ms for a speed of 4800 bps) and a pause of 3 ms is maintained. After that, the second byte is transmitted and the data transfer block is turned off until the next request. The format of the transmitted bytes is shown in fig. four.
NUM1 - the most significant digit of the LCD, NUM4 - the least significant digit, respectively. KF - coefficient by which the obtained indicator value is divided. For example, the indicator readings (-12,36) will correspond to: NUM=1, NUM2=2, NUM3=3. NUM4=6, KF=100, ZNAK=1. Relatively slow galvanic isolation optocouplers cannot operate at speeds higher than 9600 bps, although 2400 bps is sufficient in this device. The microcontroller firmware specifies a baud rate of 4800 bps. The output node of the transmission unit is made on optocouplers U1 and U2 according to a symmetrical scheme. The differing levels at pins 5 and 6 of DD5 turn on the emitting diode of one of the optocouplers. Resistors R5 and R6 are used to protect the COM port in case of improper installation or other malfunctions. The optocoupler request circuit (U3) is made according to an asymmetric circuit. Diode VD1 serves to protect the optocoupler LED from reverse voltage at the input. Now a few words about the operation of the software. The control software for the computer and the PIC controller is built in the same way [8]. Each cycle of converting numerical data from the multimeter LCD consists of the following steps. First, information is recorded (written) in the registers, then it is sequentially shifted and read into memory, all digits are inverted at a high level at pin 21 (BP) of the ADC, the sign, commas and high-order digit of the LCD are read, the remaining digits of the LCD are converted, error checking. The program for the PIC controller additionally packs data into two bytes and transfers them over a serial channel.
Instead of the optocouplers U1, U2 indicated in the diagram, you can use the dual device TLP521-2. Capacitors C2, C3 - K50-35 or other small ones. Capacitors C1, C4 - ceramic. Resistors - any, designed for surface mounting (size 1206). The type of connector XS1 depends on the extension cable used (in the diagram it is indicated for a standard printer cable). The printed circuit board is made individually for the existing multimeter model and placed inside it. Chips DD1-DD3 are mounted on the surface of the printed circuit board on both sides. On the same printed circuit board, the elements of the device shown in Fig. 3. The XP4 plug is installed directly on the multimeter body. You can use an imported analogue of the register KR1564IR9 - 74NS165 in a surface mount case. Then the DD1-DD3 microcircuits are mounted on a single-sided printed circuit board measuring 50x13 mm, and the remaining elements are mounted on a separate printed circuit board. However, due to the reduced pin pitch (1,27 mm), installation is much more complicated. In the DA1 voltage regulator, it is possible to use 78L05, KR1157EN5A or KR1157EN502A, taking into account the difference in the pin numbering. Literature
Author: V.Stepnev, Moscow
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