8085 CPU based ISSF Target Turner

My club uses pneumatic systems to turn the ISSF Targets, which are controlled by a timing system. One of the members asked me to help build a phone interface for the systems.

The systems are used for many courses of fire, and there are quite a few options to manage. On the front panel there is a RESET, which is tied to the CPU RESET, and a FACE button which returns the targets to face the shooter for scoring.

Target Turner Front Panel.

It turns out that the retired systems are based on a 8085 CPU, in the classic minimum configuration with an 8155 providing 256 Bytes of RAM, and input and output ports. There is a 2732 UV PROM holding the program.

CPU Board for the Target Turner.

So, how do we get these devices online? My thoughts are to add a serial port so that the system can be controlled remotely, then to use an additional WiFi enabled device which can present a web interface to the Range Officer to control proceedings.

Existing ROM

First step is to see what is going on under the hood here. So using the TL-866 the binary code on the ROM was read, and then using z88dk-dis the existing code could be interpreted.

It was interesting to see a very simple method of operation in the existing ROM. The system can only change course of fire if it is RESET, when it reads the position of the switches, and then halts awaiting an interrupt to trigger the course of fire. When the string is finished it will return to repeat the same course of fire.

Timing was based on a delay circuit providing 500ms of delay per unit. Perhaps it is not 100% accurate, but good enough for the application.

I believe that I found a bug that has been latent in the device for the last 40 years. It seems that an address byte was reversed, which would cause a jump into empty addresses. Not sure why no one realised that previously.

Building Serial Interface

I’m planning to build a simple serial interface which will read a character, and then change the course of fire based on that character. Initialising the course of fire can be then done by the web interface, by triggering an interrupt, or by using the wired front panel interface.

After asking the experts I learned that the SID/SOD pins on the 8085 can be used as a bit-bang serial port. In fact that is the standard way of building a serial port for early systems. The code for building serial transmission is included in the early application notes.

The serial code works perfectly at 9600 baud on this 3MHz system. Since only one character will be received and a few transmitted on boot, there are no performance issues to consider.

I’ve written the upgrade code to replicate the front panel selection process, and to allow the system to behave exactly as before when no serial input is available. When a serial command is available, which is triggered by activity on the RST6.5 line, then the system will set a different course of fire than is shown on the front panel. The string can be triggered either by the front panel, or by the interrupt related to the serial interface.

ESP-32 Web Interface

Following a bit of a search the Adafruit HUZZAH32 Breakout presented itself as the best solution to web enable the Target Turner. It can be powered by 5V, and the RX is protected against 5V input by a diode.

The physical interface is going to be a FTDI Basic style connector. Using this connector will allow me to best the 8085 first, and then build the web interface and test separately from the Target Turner. The last step will be to integrate the two devices into a system.

Using the simple serial character interface, it should be possible to present an active web page to the Range Officer.

There are many, eg, tutorials on how to build active web pages using the ESP-32 and WebSockets.

More when this is progressed further.