The Atari XL Parallel Bus Interface Part I

by Roland Scholz

This series of articles is dedicated to the so-called Parallel Bus Interface (PBI). The first part considers the several hardware signals of the PBI, the second one describes the hardware needed to form a PBI-device and the third article shows how this device is connected to the operating system by software.

As you know, our beloved Atari was developed and built by talented engineers, who had the foresight to give it the ability to connect up to eight devices on a parallel bus.

Yet where can that parallel bus be found and what sort of devices use it? If you take a look at the back of the Atari 600XL or 800XL, you will see an ominous lid labelled, "PARALLEL BUS". If the lid is removed 50 silver pins, 25 on each side, will smile at you. And they emit something else: performance! As the term "parallel bus" suggests, the data are not tranferred serial, bit by bit, but parallel, which means 8 bits at a time. This is not even done at the pace of 19,200 baud, as the serial bus does, but at the maximum pace of the 6502 CPU. However, this printed circuit board connector is directly linked to most important signals of the CPU and contains signals to embed a device in a plug & play manner. So now it is time to take a closer look at the pins as they are shown in figure 1 (see left):

In the following description the backslash (\) denotes a signal, which is active when the wire has a voltage of 0V or, in the diagram, a bar is used to show this.

The data-flow directions "in" and "out" are seen from the CPU or from the bus, respectively.

A0-A15, out:
These are the address signals of the processor. Every time data are transferred over the data bus, the CPU signals with these wires which storage location is meant to transmit or receive data. Since there are 16 address signals and each signal can assume two states (0V or +5V), the Atari is able to address 2^16 = 65,536 locations of storage.

D0-D7, in-out:
The effective data are transferred using these lines. Since there are 8 of these lines, our Atari belongs to the so-called 8-bit computers.

Reset\, out:
When the Atari is switched on or the reset button is pressed, this line goes low (0V) for a short time. Because of that, many of the internal chips are initialized and the processor starts to execute machine code that begins at the address stored in the location $FFFC-$FFFD. Usually the operating system (OS) ROM covers this area.

PHI2, out:
This line provides for the system clock of the computer. It controls how long a memory access takes and when it is finished. PHI2 oscillates with a frequency of roughly 1.77 MHz, so one oscillation takes only 1/1,770,000 = 0.565 microseconds. In the first half of the oscillation PHI2 drives low level, it switches to high level during the second half. That means all memory accesses have to be finished by the falling edge of PHI2.

LR/w, out:
This signal is called "latched read/write". It is generated by the CPU to show if the actual memory access is a read or a write access and it is latched internally by the Atari hardware. If the LR/w drives high level the CPU is to read data from memory, in the other case the data are to be written from to CPU to memory.

IRQ\, in (interrupt request, open collector)
This signal tells the CPU it is to be interrupted. Due to this line, the CPU or the program, respectively, does not have to poll a certain line, but the currently running program will be stopped, the CPU registers are saved and the CPU starts to execute the program whose starting address is stored at $FFFE-$FFFF. As you might expect, these locations are covered by the OS as well, so that the OS first takes control in order to see whether there are keys pressed or data are to be transferred over the serial bus (floppy, printer...). When that is done, the CPU resumes execution of the interrupted program.

RDY\, in:
This line gives an opportunity to stop the CPU for a short time. Since the custom chip ANTIC heavily uses this line as well, it is difficult to use this line without disturbing both chips.

MPD\, in:
This signal is called "math pack disable" and is used by an external hardware to switch off the floating point routines located in a ROM that reaches from $D800-$DFFF (2KB). If this is done, you are free to put another ROM containing drivers or peripheral hardware there. However, when your own ROM is switched in, you cannot use the floating point routines, if you do not want your system to crash.

EXTSEL\, in:
If this line is held low during a memory access, the internal RAM of the Atari is not accessed. This signal is usually used in conjunction with MPD\, so the math-ROM and the internal RAM are deselected and a user ROM may be accessed for that cycle. Another usage is the 64KB expansion for the Atari 600XL, which turns off the internal RAM using this line.

CasInh\, out:
This signal drives low level, if the Atari performs a RAM access. Otherwise, if ROM or I/O chips are selected, this signal will go to high level.

REF\, out:
The ANTIC chip is responsible for refreshing the dynamic RAM every few milliseconds. If the actual memory is a so-called refresh cycle, this signal will show low level.

RAS\, CAS\, out:
The internal structure of dynamic RAM is formed like a matrix. Each storage cell is addressed by row and a column. So the Atari has to split its 16-bit address into two addreses for the internal matrix and generate these two strobe signals (row addres strobe, column address strobe), so the RAM knows when each address is valid.

AUDIO IN, in:
With this line, you can mix your own audio signal to the sound generated by the POKEY chip.

Now all the signals of the parallel bus interface have been described briefly. The next part of this series will show how these signals can be used to form a device that is noticed and initialized by the Atari during the power-up procedure. If there are questions or suggestions, please write a couple of lines to roland_scholz@web.de

MyAtari magazine - Feature #10, May 2001