Signetics 2650 & 2636 programming/Console Hardware

Externally these consoles are very similar to one another apart from their styling. They all have a slot for the game cartridge, four switches on the front, a pair of hand-controllers, and leads for power input and connection to a TV. Some have external power packs that supply low voltages to the console from the mains, while others perform this function internally.

The four switches on the console are power on/off, reset/load, start and select. (The load label that appears on some machines has led to some misinformation appearing on the web, as it has given the impression that the game is loaded from the cartridge to internal memory. This is incorrect as there is no internal memory on these machines. That button simply forces the microprocessor to start executing the program at memory address $0000 in the cartridge.)

Some of the consoles, such as the Database, have all the electronics on one printed circuit board, while others such as the Interton have divided the circuit into modules housed on smaller boards.

The game cartridge was configured in several different formats. Most had either a 2k or a 4k ROM, while some were 6k ROM and some had 1k RAM. The Acetronic Hobby Module was 2k ROM and 2k RAM.

System architecture
This block diagram shows the main components of the console and their most significant interconnections:

Clock generation
All the necessary clocks for the system are derived from a single 8.867238 MHz crystal connected to the the video encoder. The encoder uses this for generating colour signals in its composite video output. It also divides it by 2½ to give a 3.55 MHz clock for the 2621 sync generator.

The sync generator passes this to the 2636 PVI as a pixel (or position) clock, and divides it by four to clock the microprocessor at 887 kHz. It also generates the vertical and horizontal reset signals for the PVI, as well as composite video timing signals for the video encoder.

Processor
The Signetics 2650 is an eight bit microprocessor capable of addressing 32kB of memory. However, in this console only 13 of its address lines are used to access up to 8kB of memory.

The Reset signal causes it to start executing code at address $0000.

An $\overline{Interrupt}$ signal from the PVI, in conjunction with a vector supplied by the PVI, can cause execution to temporarily switch to $0003.

The Sense input is used to detect the state of the VRST signal (vertical blanking). The Flag output is used to determine which of the joystick potentiometers are connected to the PVI.

2636 PVI
The 2636 Programmable Video Interface has 108 registers that control its audio and video output. Four other registers provide the processor with information about video object collisions and analogue joystick data. A further 37 registers are available as general purpose memory locations.

The PVI also generates chip select signals for various interface circuits and memory in the games cartridge. It also sends and interrupt request signal to the processor when it needs attention. See 2636 PVI for more details.

Effects
The PVI can only generate a single square wave frequency. This is fed to some audio effects circuits that are controlled from the microprocessor via the 74LS378 latches. These circuits:
 * turn the PVI audio on/off
 * turn on white noise
 * cause an explosion sound
 * set one of four volume levels

One other bit in the effects register can invert the colour of the background grid and screen.

Player inputs
The Reset button on the console causes the processor to start executing the program at memory address $0000 in the game cartridge.

The Start, Select and keypad buttons are all controlled by the 74LS258 and 74LS156. They are arranged as a matrix, with the '156 selecting the column, and the '258 reading the data from the rows.

Each joystick controls two variable resistors. The four variable resistors are connected to the two analogue-to-digital converter inputs on the PVI via the CD4053 analogue multiplexer. The Flag output from the microprocessor controls which two resistors are to be measured.

Programmers should note that the joysticks are self-centring on some consoles and not on others. This can have a big impact on how they are used and programmed.