Electronic event recorder. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Commercially available electronic event recorders are mainly designed for industrial applications and are quite expensive. The proposed device can be made independently. It does not contain expensive parts and captures the operation of four sensors, contact or with output signals of standard logic levels.

Fig. 1

The diagram of the recorder is shown in fig. 1. Its basis is the PIC16F628A (DD1) microcontroller. Information about events (changes in the state of contact sensors SF1-SF4) is stored in the DS1 non-volatile memory chip connected to the microcontroller via the l2C serial interface.

The choice of the PIC16F682A microcontroller is due to the following:

- there is a built-in USART for organizing communication with a computer;
- low power consumption;
- operability is maintained when the supply voltage drops to 2,2 V;
- there is a timer synchronized by a separate quartz resonator, convenient for organizing a real-time clock and working even in the "sleep" mode of the microcontroller.

Information about each event occupies eight bytes in the memory of the DS1 chip. This means that the available 32 KB is enough to register 4096 events. Reading and writing are performed block by block: the address of the first byte of the block is followed by eight bytes of information. The increment (increase by one) of the address of each received or transmitted byte occurs automatically.
Most of the time, the microcontroller DD1 is in sleep mode, only its timer TMR1, synchronized by the "clock" quartz resonator ZQ2, works. Once per second, this timer generates an interrupt request, to process which the microcontroller wakes up, increments the time counter and polls the sensors. If their state has changed, the event information is written to the DS1 chip, after which the microcontroller "falls asleep" until the next interrupt.
An event registration (with indication of the state of the sensors, time and date) results in a change in the state of any of the four sensors. But you need to keep in mind that the sensors SF1 and SF2 are polled once per second, so short-term changes in their state may be missed. This can be used to eliminate the effects of contact bounce. But changes in the state of sensors SF1 and SF3 will be recorded, even if they occurred in the intervals between polls, and by the time of the poll the sensor has returned to its original state. These features should be taken into account when choosing the design of sensors and the method of their connection.

The current consumption of the recorder from a 3 V battery is about 200 μA, which allows it to operate from two AAA alkaline cells for at least six months.

To interact with a computer, the recorder has a serial RS-232 interface, the X1 connector of which is connected to the computer's COM port connector.

This allows:

- transfer the list of registered events from the device to the computer;
- clear this list in the registrar;
- set the exact time in the recorder's clock.

The RTS signal generated by the computer's COM port was not used for its intended purpose, but to power the DA1 level conversion chip. This reduces the current drawn from the GB1 battery and extends its life. The supply voltage of the DA1 chip is limited by the VD1 zener diode to 3,3 V. This is less than the typical value for the applied MAX232CPE chip, but it is sufficient for its normal interaction with a physical or virtual (organized using a USB-RS-232 converter) computer COM port.
A voltage of 3,3 V from the RTS line is also supplied to the HL1 LED, which indicates its presence, and to the RA2 input of the microcontroller. 

Table 1

Note. Time is transmitted in six bytes in the following order, seconds, minutes, hours, day, month, year.

Upon detecting it, the microcontroller wakes up and initializes its USART. The exchange is carried out at a speed of 19200 baud.
The microcontroller is set to its initial state when the power is turned on by an internal signal. The MCLR input is not used.

Internal resistors are programmatically connected to the lines of port B of the microcontroller, connecting them to the power plus. They provide a current flowing from each line of about 200 µA. Event sensors can be not only physical contacts (reed switches, buttons), but also any devices with output logic signals that have standard TTL levels.
Socket X1 - DB-9F. As X2, any convenient connector can be used to connect sensors. When connecting the recorder to devices that have their own power sources, especially network ones (for example, to fix power outages, equipment failures), you need to take care of galvanic isolation using optocouplers or relays.
To interact with the logger, a computer program logger.exe has been developed that allows you to give it the commands listed in the table, receive responses to them and save the list of registered events in a file on the computer disk. The window of this program is shown in Fig. 2.

Fig. 2

When the recorder is connected, it displays the number of events recorded by it, as well as the current time and date according to the recorder's internal clock.

By opening the "Journal" main menu item, you can synchronize the clocks of the recorder and the computer, save the list of events in a CSV (Comma Separated Values) file, and clear the recorder's memory. The CSV format is chosen as the most versatile, such a file can be opened, for example, in Microsoft Excel or in another spreadsheet. It is easy to write your own program for processing it.

The event record format is as follows: day, month, year, hour, minute, second, sensor 1, sensor 2, sensor 3, sensor 4. All values ​​in it are decimal text. The character separating them (in this case, a semicolon) is set in the window that opens when you select the "Options" menu item. In the same window, select the COM port of the computer to which the recorder is connected, and the period for polling it by the computer (not to be confused with the period for polling sensors by the recorder).

The program can be downloaded hence.

Author: S. Kuleshov, Kurgan; Publication: radioradar.net

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