Computer Buses

Martin Russ continues his look at computer buses and reports back from the recent 1996 Apple Expo show.

After last month's special feature where Nick Rothwell talked about his use of Macs and Max in live performance, Apple Notes returns to the subject of interface buses.

• PDS
  Processor Direct Slots (PDS) connect directly onto the Address and Data buses of the main 680n0 processor in the Mac. PDS slots can be found in the lower cost 680n0‑based machines (the Mac II, Quadra, Centris, Performa ranges), but they vary depending on the type of Mac — particularly for the different instruction sets of different processors. 68000, 68020, 68030 and 68040 variants all exist. Connecting anything directly to the Address and Data buses has inherent risks — particularly since anything that goes wrong on the PDS card can stop the main processor from functioning. Adaptor cards are available which can convert some PDS slots into NuBus slots.
• SCSI
  The Small Computer Systems Interface is intended for use both inside and outside a computer. When inside the case, Macs use it for the internal communications between processor and hard drives (some PowerMacs use SCSI‑II, a dedicated faster version which optimises the access to the hard disk). Outside the computer, it is used for connecting additional hard drives (both opticals and removables), as well as scanners, CD‑ROM drives and other peripherals. It provides addressing for up to seven peripheral devices, with the Mac processor taking the eighth address. Data rates of up to 5Mb per second are supported by the basic SCSI specification, but this depends on the hardware, software and cable lengths. There are many variants of SCSI, including SCSI‑II, Wide SCSI, and Fast & Wide SCSI.
• NuBus
  The NuBus is a high speed bus with a 20Mb per second maximum transfer rate. NuBus slots are found on the more expensive Macs, and provide buffered access to the processor address and data bus. There are two types of NuBus slots — electrically identical, but with different physical lengths: short and long. The short NuBus card is approximately the same size as the short VGA/Printer/Mouse cards which are typically found in an IBM PC‑compatible computer. The NuBus connector is a 96‑way, DIN C096M connector. NuBus cards can be up to 4 inches in height, and between 12.875 and 7 inches long.

The NuBus is optimised for 32‑bit Address and Data transactions, but it can also be used for 8‑ and 16‑bit nonjustified transfers. It uses a synchronous bus, where all the transactions (Reads and Writes) are determined by a master system clock, but it has many of the features of an asynchronous bus, and transactions may be a variable number of clock periods long. The master clock runs at 10MHz in most Macintosh models, and is supplied by the Macintosh. I/O and Interrupts are memory mapped, and there is a single, large address space used for accessing all the NuBus slots in a computer.

No DIP switches or jumpers are needed to set up NuBus cards — geographical addressing and distributed parallel arbitration eliminates slot problems. You can put the card in any slot and the system will find it! This is Mac 'plug and play', and it was first introduced about nine years ago! Firmware on the card identifies it to the Macintosh's operating system, and the software driver for the NuBus card can also be stored as part of the ROM firmware. The data transfers require 75ns setup and propogation times. NuBus cards can also be used as Video cards for the Mac, in which case the Mac's internal video RAM is replaced by that on‑board the NuBus card.