From Wikipedia, the free encyclopedia

Released as the expansion bus of the Commodore Amiga 3000 in 1990, the Zorro III computer bus was used to attach peripheral devices to an Amiga motherboard. Designed by Commodore International lead engineer Dave Haynie, the 32-bit Zorro III replaced the 16-bit Zorro II bus used in the Amiga 2000. As with the Zorro II bus, Zorro III allowed for true Plug and Play autodetection (similar to the PC's PCI bus) wherein devices were dynamically allocated the resources they needed on boot.

Zorro III continued Zorro II's somewhat controversial direct-address design, which was almost impossible to access as ported I/O and ate into CPU address space, a limiting factor for CPUs with 24 bit addressing. The CPU could directly address any Zorro III device as memory, so Zorro memory expansions could be made (and were made) as well as it being possible to use video memory on a video card to be as system RAM.

As an asynchronous bus, Zorro III specified bus cycles of set lengths during which a transaction conforming to the specifications of the bus could be carried out. Unlike PCI which relied on an arbiter to assign the bus, Zorro III used a bus terminator to perform the same function, the Buster (Bus Terminator) chip in Amiga 3000 and Amiga 4000. This meant that PCI could scale more smoothly (but other limits prevented more than 7 devices per bus on PCI) but that Zorro III had a lower latency and was faster at servicing interrupts.

Despite being a 32-bit bus, Zorro III used the same 100 way slot and edge connector as Zorro II. The extra address and data lines were provided by multiplexing some of the existing connections with the nature of the lines changing at different stages of the bus access cycle (e.g. address becoming data). This allowed the use of Zorro II expansion cards on the newer bus, coexisting with Zorro III cards. Certain Zorro III cards would also function when connected to the older Zorro II bus, though at Zorro II's reduced data rates.

Although the Zorro III bus has a theoretical bandwidth of 150 MByte/s (32-bit x 37.5 MHz), the real transfer speed between the system and a Zorro III card is less than 20 MByte/s due to the limitations of the Buster chip. This is also the limiting factor with PCI expansion boards like e.g. Elbox Mediator PCI or the Matay Prometheus PCI (about 12 MByte/s PCI to 68k-system). DMA transfers between two Zorro III cards (or PCI cards on an PCI expansion board) can be much faster. [1], [2]

Memory map

Physical

The physical connector is a standard 2,54 mm spaced (100 mil) card edge connector with 2 x 50 rows of pins. ^{[3 ]}

Power:

^[2]

References

1. ^ "The Amiga 3000+ System Specification An enhanced Amiga 3000 family computer Document Revision 0.6 1991 DevCon Release by Dave Haynie July 17, 1991 Copyright 1991 Commodore-Amiga, Inc.". http://www.thule.no/haynie/research/a3000p/docs/a3000p.pdf.   090501 thule.no p14
2. ^ ^{a b} "The Zorro III bus specification. A General Purpose Expansion Bus for High Performance Amiga Computers. Document Revision 1.10. by Dave Haynie March 20, 1991 Copyright 1990, 1991 Commodore-Amiga, Inc.". http://www.thule.no/haynie/zorroiii/docs/zorro3.pdf.   090430 thule.no p16 fig1-1
3. ^ "Replacement zorro slots? - English Amiga Board". http://eab.abime.net/showthread.php?p=411339.   090501 eab.abime.net

External links

• thule.no - The Zorro III Bus Specification (PDF)
• List of device bandwidths - Notes on Zorro III performance
• amiga.resource.cx - Amiga Hardware Database Descriptions and photos of Zorro III cards
• amigahardware.mariomisic.de - The Big Book of Amiga Hardware