Parallel Bus Interface

The Parallel Bus Interface or PBI is a 50-pin port found on some Atari 8-bit XL computers. It provides unbuffered, direct connection to the system bus lines (address, data, control), running at the same speed as the 6502 CPU.

Only the 600XL and 800XL computers had a PBI. XE systems came with the Enhanced Cartridge Interface (ECI), a functionally similar connection.

Details

During coldstart, the Atari OS polls for various parallel bus devices. If character devices are found, it'll load handler (aka driver) information into its handler table and initializes the parallel bus device, if needed.

Instead of loading a driver to RAM, a character device can utilize the Generic Parallel Device Handler that's already provided (at a fixed address in ROM) once the address is loaded into the handler table. The device connected to the parallel bus gets its ROM space mapped to the OS Floating-point package subroutines and tables. While the device is selected, no floating-point operations can take place. Once de-selected, the floating-point routines are restored.

A block device does not use the handler table, the only thing needed for proper initialization is setting a predefined flag, that the device is present. A block device can not use a softloaded driver, its driver must reside in the aforementioned ROM space.

Up to eight devices can be connected at a time. When they have been initialized properly, the OS selects them and deselects automatically as appropriate.

Pinout

Parallel Bus Interface (PBI) - looking at the back
1	3	5	7	9	11	13	15	17	19	21	23	25	27	29	31	33	35	37	39	41	43	45	47	49
2	4	6	8	10	12	14	16	18	20	22	24	26	28	30	32	34	36	38	40	42	44	46	48	50

Parallel Bus Interface (PBI) - Pinout
Pin	TOP	Pin	BOTTOM
1	GND Ground	2	External select
3	A0 Address output	4	A1
5	A2	6	A3
7	A4	8	A5
9	A6	10	GND
11	A7	12	A8
13	A9	14	A10
15	A11	16	A12
17	A13	18	A14
19	GND	20	A15
21	D0 Data (bidirectional)	22	D1
23	D2	24	D3
25	D4	26	D5
27	D6	28	D7
29	GND	30	GND
31	Phase 2 clock output	32	GND
33	NC Reserved	34	Reset output
35	(IRQ) Interrupt request	36	Ready input
37	NC	38	External decoder output
39	NC	40	Refresh output
41	Column address output	42	GND
43	MPD Math pack disable input	44	Row address strobe
45	GND	46	Latch read/write out
47	NC (+5V on 600XL only)	48	NC (+5V on 600XL only, used for 1064)
49	Audio input	50	GND

Devices

• Atari 1064 Memory Module - Designed specifically for the 600XL, it upgraded the memory of the 600XL to 64kB. Only parallel bus device released by Atari.

• Atari 1090 Expansion System - Designed to give expansion slots to the XL computers. Never released.

• ICD Multi I/O Board - Hooked up to either a PBI or ECI connection. Featured a RAMdisk, parallel port, serial port, print spooler and a hard disk interface.

• Black Box - The Black Box by CSS hooks up to either PBI or ECI connection. Featured a SCSI interface, a serial port, parallel port, floppy drive interface and a built-in menu system.

See also

• Enhanced Cartridge Interface (ECI)

References

• Chadwick, Ian (1985). Mapping the Atari Revised Edition. COMPUTE! Publications, Inc. ISBN 0-87455-004-1. [1]

External links

• ICD Boosts 8-bit Atari Power Antic Magazine. Review on ICD products for the Atari.