STEbus

The STEbus is a non-proprietary, processor-independent, bus with 8 data lines and 20 address lines. It was popular for industrial control systems in the late 1980s and early 1990s before the ubiquitous IBM PC dominated this market.

It remains a well-designed standard. Although no longer competitive in its original market, it could be a valid choice for hobbyists wishing to make 'home brew' computer systems. The Z80 and probably the CMOS 65C02 would be good processors to use. The standardised bus would allow hobbyists to interface to each others designs.

Origins

In the early 1980s there were many proprietary bus systems, each with its own strengths and weaknesses. Most had grown in an ad-hoc manner, typically around a particular microprocessor. The S-100 bus is based on Intel 8080 signals, the STDbus around Z80 signals, and the G64 bus around 68000 signals.

This made it harder to interface other processors. Upgrading to a more powerful processor would subtly change the timings, and timing restraints were not always tightly specified. Nor were electrical parameters and physical dimensions. They usually used edge-connectors for the bus, which were vulnerable to dirt and vibration.

The VMEbus had provided a high-quality solution for high-performance 16-bit processors, using reliable DIN41612 connectors and well-specified Eurocard board sizes and rack systems. However, these were too costly where an application only needed a modest 8-bit processor.

In the mid 1980s the STEbus standard addressed these issues by specifying what is rather like a VMEbus simplified for 8-bit processors. The bus signals are sufficiently generic so that they are easy for 8-bit processors to interface with. The board size was usually a single-height Eurocard but allowed for double-height boards as well. The latter positioned the bus connector so that it could neatly merge into VME-bus systems.

Maturity

The STEbus was very successful in its day. It was given the official standard IEEE1000-1987, and supported processors from the popular Z80, the 6809, to the powerful 68020. The only popular micro notably absent was the 6502, because it did not naturally support wait-states while writing. The CMOS 65C02 did not have this shortcoming, but this was rarer and more expensive than the NMOS 6502 and Z80.

Peripheral boards included prototyping boards, disc controllers, video cards, serial I/O, analogue and digital I/O.

The STEbus achieved its goal of providing a rack-mounting system robust enough for industrial use, with easily interchangeable boards and processor independence.

Decline

The STEbus market began to decline as the IBM PC made progress into industrial control systems. Customers opted for PC-based products as the software base was larger and cheaper. More programmers were familiar with the PC and did not have to learn new systems.

Memory costs fell, so there was less reason to have bus-based memory expansion when one could have plenty on the processor board.

So despite the disadvantages, manufacturers created industrial PC systems and eventually dropped other bus systems.

Moving on, PC systems did away with the need for card cages and backplanes by moving to the PC104 format where boards stack onto each other. While not as well designed as the STEbus, PC104 is good enough for many applications.

The major manufacturers from its peak period now support STEbus mostly for goodwill with old customers who bought a lot of product from them.

The IEEE have withdrawn the standard, not because of any faults but because it is no longer active enough to update.

Connector

DIN41612, rows a and c, 0.1" pitch.

Physical format

3U Eurocard - The most common size was the 100 x 160 mm Eurocard.

6U Eurocard - Rare, sometimes used in VMEbus hybrid boards

External links

* [http://www.howell1964.freeserve.co.uk/Arcom/STEbus.htm STEbus connector and signal description]
* [http://www.howell1964.freeserve.co.uk/Arcom/ArcomManuals.htm STEbus boards and manuals]
* [http://standards.ieee.org/reading/ieee/std/busarch/1000-1987.pdf STEbus (IEEE1000) standard (requires payment)]


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