After writing the previous post, I realized that I was at a point where I could actually test one of the memory board devices, the real-time clock. This chip will allow LEO-1 to keep track of the date and time. It supports a battery backup so it can remember the date and time even when the power is turned off. Since all this was already soldered up and I had tested the address decoding, I put the chip in its socket, set up the test probes and set my test DIP switches to the address of the chip. Then I switched the test switch called /OE to off, which is the signal to the addressed unit that it must be output-enabled and put its data on the data bus. I switched on the power and the LEDs on the breadboard started flashing… 0001, 0010, 0011, 0100… a sign that the real-time clock was outputting the seconds onto the data bus as it should. It was working!
The chip I chose is the RTC 72421 which is made by Epson. The data sheet for this is one of the good ones; it’s really well written and explains everything completely. I was able to use my test board to visit each of the chip’s 16 registers and set it to 24-hour mode, and set the date and time. After that I put the battery in and switched off the main power. The chip outputs a pulse called STD.P on pin 1 which works even when it’s on standby. I set it to one-second-per-pulse mode. You can feed it to an LED driver or a piezo speaker to hear the clock ticking. I plan to leave the battery in overnight and check in the morning if it kept the right time. You can tell that I’m nuts about dates and times. That’s one of my ‘special things’.
So, my first proper test was successful. That’s a good sign.