Program for operating the buttons on the EPS-103 unit (and controlling the motors)

It is great to be able to write any inscription on the display. EPS-103 can tell us what our program is doing. Another peripheral that needs to be mastered is the keyboard. The EPS-103 has a total of 16 switches. Above all, two large buttons Up and Down. Under the opening door we find buttons 0 to 9 and button E. And a three-position MODE switch, which actually works as three separate switches, one of which is always permanently closed.

The 16 switches are wired into a 4 x 4 wire matrix. The input to the matrix is connected to port A of circuit 8255A, i.e. at address 5800h. Only the bottom half of the port is active. The upper four bits are unused according to the scheme. Here would be the option to expand the keyboard with additional user buttons. The output from the matrix is directed to port B of the 8255A circuit, i.e. to address 5801h. Again, only the lower four bits are active. On the top half of the port, internal jumpers W1, W2 and W3 labeled Test are connected according to the diagram.

Using the matrix keyboard is quite simple. The binary number 1110 (1101, 1011, 0111) is written to the input and then the output is read from the matrix. According to its binary value (1111, 1110, 1101, 1011, 0111), we know which switch was activated. The value 1111 means that no button is pressed. The following table shows the values of each switch. An asterisk replaces bits that do not affect the function of the keyboard. The program usually overwrites them with all ones.

The RUN switch is wired a little differently than the other switches. It is in series with the Up and Down buttons. This means that using the PRG or TCH switch disables the Up and Down buttons. Therefore, it is necessary to first identify these three buttons and the Up and Down buttons with them in the first loop. Only subsequently in the second loop do we scan the remaining buttons. After identifying the pressed key, we can start an activity. For example, to write on the display which key is pressed.

This program will need RAM to run. We will use the space allocated to the stack. The stack starts at address 4700h and fills towards lower values. That's why I chose the addresses 4600h to 4603h. They're pretty close to the stack so the original firmware definitely doesn't use them for anything else. At the same time, they are far enough from the beginning of the stack that our program does not overwrite them with other data when using the stack.

At address 4600h, we write the code of the pressed key from the trio PRG, TCH, RUN. If any of the remaining keys is pressed, we write its code at address 4601h. It only makes sense to print the name of the pressed button on the display when it has changed. Therefore, control addresses 4602h and 4603h are also used. We copy the values of the main addresses 4600h and 4601h into them. By comparing the originals and copies, we can see if the status of the keyboard has changed.

Keyboard scanning can be part of the main program. But usually repeated actions are handled by some kind of interrupt. The EPS-103 has a generated HW interrupt RST7.5 from the NE555 pulse generator. The pulse period and thus the interruption repetition is 4 ms. This could be used to operate the keyboard. We just need to make sure that our handler finishes before the interrupt fires again. For example, clearing the LCD display takes 15ms according to the documentation. This means that we will not be able to operate the display during the interruption.

For study reasons, we will use the RST7.5 interrupt, but only to scan the keyboard. We write the result of the scan into RAM at addresses 4600h and 4601h. There will be a loop in the main program that will read these addresses and perform processing based on the contents, including writing to the display.

The introduction of the program is the same as the program Hello world. We first set the stack address in RAM to 4700h and then initialize the 8255A circuit. Then we need to set the interrupt correctly. RST7.5 needs to be enabled by writing 0 to bit 2 of the interrupt mask register. This is done by the SIM instruction. Then we can trigger the interrupt with the EI instruction. The infinite loop will repeatedly read addresses 4600h and 4601h and compare them to the copies at addresses 4602h and 4603h. If they differ, we first update the copies. Then we erase the entire display and write the setting of the MODE switch on it, followed by the name of the pressed button. The program does not handle the possibility of pressing multiple buttons at the same time.

In order for the program to have a specific function after all, we can activate the motors in the corresponding direction by pressing the 2/E, 3/U, 4W and 8/D buttons. After releasing the button, we stop the engine. If someone modifies the program to look for more simultaneous button presses, they have to think carefully about how to respond to them. For example, it's fine to run both the azimuth and elevation motors at the same time. It makes no sense to activate the rotation of one motor on both sides at the same time. Changing the direction of rotation while the motor is running would likely damage the motor.

The motors interface is connected to port C of circuit 8255A, i.e. at address 5802h. The motors are controlled by writing a control byte to this port. The meaning of individual bits is as follows:

 A corresponding program might look like this:

The resulting binary file is loaded by the programmer into the erased EPROM memory and can be tested. I believe the information provided in this program will help anyone who would like to research the original EPS-103 firmware. You can download the source text of the mentioned program in the file KeyboardEPS103.asm..