June 1998, Issue 95

Gotchya!
Alarming the Alarm System

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Alarm-System Data Logger

Figure 2 is the schematic of the alarm-system data logger (ASDL). The circuitry was added to Domino’s proto1 board, as shown in Photo 2. The first task was mating the 12-V level alarm signals with Domino’s TTL input levels.

(Click here to enlarge)

Figure 2– The Prototype ASDL system is physically assembled using a Domina development board with an LCD and keypad attached.

 

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Photo 2–The prototype ASDL system is physically assembled using a Domino development board with an LCD and keypad attached.

There are lots of ways to do this, but one cute way is to use MC1489 RS-232 input level shifters. These inexpensive inverters can withstand ± 30-V inputs while interfacing directly with TTL on the output side. Six keypad lines connect to the processor.

The next detail is determining when a key is pressed. We had two alternatives. We could create a falling-edge key-press interrupt by NORing the three column inputs together. Or, we could take advantage of the fact that pressing any key caused the beeper to sound.

Ultimately, Jeff chose to use the signal applied to the beeper as a key-press interrupt. The three column lines, two phase outputs, and on/off button are read anytime there’s an interrupt. Together, they determine the physical key combination.

Normal entry and exit profiles run a total of 10 key presses a day. This plus a 7-byte time and date stamp amounts to fewer than 20 bytes of data-logging space required per day.

Our plan isn’t to check this thing daily but to have a record of events available when we need it. If we have a good chunk of data storage space available, how often we dump the records will never really be an issue. Six months of data fits easily in less than 5 KB of memory. We have about 24 KB available.

Program development started with writing the time and date to the LCD. Instead of using a couple I2C peripheral chips to connect the 4 ´ 20 LCD and 4 ´ 6 keypad as typically described for Domino, Jeff chose to minimize external hardware and scan the command keypad in software.

Using this number of I/O bits for the keypad necessitated using the LCD in 4-bit mode rather than the typical 8-bit parallel interface. The 4-bit mode requires two nibble writes to the LCD for each printed character, which entails eight physical operations for each character—set up the control register, raise the strobe line, set up the data register, drop the strobe line. The process repeats for the second data nibble.

The program executes a short initialization routine and then prints day of the week (DOW), month/day/year, and hour/minute/second on the LCD. Using the row and column positioning capability of the LCD (a control register routine), only new data needs to be updated. This makes a great idle screen that also indicates the system is running.

If the displayed time and date is incorrect, as it would be on initial powerup, the user enters the present DOW, month, day, year, hour, minute, and second. Maintaining the time during a power outage is a simple matter of adding a 4.5-V back-up battery to the real-time clock.

Since the alarm system is normally battery-backed and may be needed during a power outage, we decided to battery-back the whole Domino2 circuit. Besides keeping the RTC alive, it maintains any collected data that hasn’t been printed yet. Figure 3 gives an overall view of how the complete system was integrated.

(Click here to enlarge)

Figure 3– The ASDL attaches in parallel with the existing alarm keypad. The Domino2 based monitor registers and records keypad activity and outputs the data to an LCD and printer on command.