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The STEbus (also called the IEEE-1000 bus) is a non-proprietary, processor-independent, computer bus with 8 data lines and 20 address lines. It was popular for
industrial control system An industrial control system (ICS) is an electronic control system and associated instrumentation used for industrial process control. Control systems can range in size from a few modular panel-mounted controllers to large interconnected and in ...
s in the late 1980s and early 1990s before the ubiquitous IBM PC dominated this market. STE stands for STandard Eurocard. Although no longer competitive in its original market, it is valid choice for hobbyists wishing to make 'home brew' computer systems. The
Z80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples were ...
and probably the CMOS
65C02 The Western Design Center (WDC) 65C02 microprocessor is an enhanced CMOS version of the popular nMOS-based 8-bit MOS Technology 6502. The 65C02 fixed several problems in the original 6502 and added some new instructions, but its main feature ...
are possible processors to use. The standardized bus allows hobbyists to interface to each other's 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S-100 bus The S-100 bus or Altair bus, IEEE 696-1983 ''(withdrawn)'', is an early computer bus designed in 1974 as a part of the Altair 8800. The bus was the first industry standard expansion bus for the microcomputer industry. computers, consisting of p ...
is based on
Intel 8080 The Intel 8080 (''"eighty-eighty"'') is the second 8-bit microprocessor designed and manufactured by Intel. It first appeared in April 1974 and is an extended and enhanced variant of the earlier 8008 design, although without binary compatibil ...
signals, the
STD Bus The STD Bus is a bus (computing), computer bus that was used primarily for industrial control systems, but has also found applications in computing. The STD Bus has also been designated as STD-80, referring to its relation to the Zilog Z80 series ...
around
Z80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples were ...
signals, the SS-50 bus around the
Motorola 6800 The 6800 ("''sixty-eight hundred''") is an 8-bit microprocessor designed and first manufactured by Motorola in 1974. The MC6800 microprocessor was part of the M6800 Microcomputer System (latter dubbed ''68xx'') that also included serial and para ...
, and the
G64 bus Graham's number is an immense number that arose as an upper bound on the answer of a problem in the mathematical field of Ramsey theory. It is much larger than many other large numbers such as Skewes's number and Moser's number, both of which a ...
around 6809 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 VMEbus (Versa Module Europa or Versa Module Eurocard bus) is a computer bus standard, originally developed for the Motorola 68000 line of CPUs, but later widely used for many applications and standardized by the IEC as ANSI/IEEE 1014-1987. ...
had provided a high-quality solution for high-performance
16-bit 16-bit microcomputers are microcomputers that use 16-bit microprocessors. A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two mo ...
processors, using reliable
DIN 41612 DIN 41612 was a DIN standard for electrical connectors that are widely used in rack based electrical systems. Standardisation of the connectors is a pre-requisite for open systems, where users expect components from different suppliers to operat ...
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 VMEbus (Versa Module Europa or Versa Module Eurocard bus) is a computer bus standard, originally developed for the Motorola 68000 line of CPUs, but later widely used for many applications and standardized by the IEC as ANSI/IEEE 1014-1987. ...
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 (100 mm x 160 mm) but allowed for double-height boards (233 x 160 mm) as well. The latter positioned the bus connector so that it could neatly merge into VME-bus systems. IEEE Working Group P1000 initially considered simply repinning the
STD Bus The STD Bus is a bus (computing), computer bus that was used primarily for industrial control systems, but has also found applications in computing. The STD Bus has also been designated as STD-80, referring to its relation to the Zilog Z80 series ...
, replacing its card edge connector with the DIN41612 connector. But they decided to create a completely new high-performance 8-bit bus. They decided to make a bus more like the VMEbus and Futurebus. The STEbus was designed to be manufacturer independent, processor independent, and have multimaster capability.ISO/IEC 10859: 8-bit backplane interface: STEbus and mechanical core specifications for microcomputers
1997. p. 4


Maturity

The STEbus was very successful in its day. It was given the official standard
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a 501(c)(3) professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operat ...
1000-1987. Many processors were available on STEbus cards, across a range of price and performance. These boards included the Intel 8031, 8085,
8088 The Intel 8088 ("''eighty-eighty-eight''", also called iAPX 88) microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers a ...
, 80188; the National Semiconductor 32008 and
32016 The NS32000, sometimes known as the 32k, is a series of microprocessors produced by National Semiconductor. The first member of the family came to market in 1982, briefly known as the 16032 before becoming the 32016. It was the first 32-bit general ...
; the
Motorola 6809 The Motorola 6809 ("''sixty-eight-oh-nine''") is an 8-bit computing, 8-bit microprocessor with some 16-bit computing, 16-bit features. It was designed by Motorola's Terry Ritter and Joel Boney and introduced in 1978. Although source compatible wi ...
,
68000 The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Secto ...
, and 68008; The Zilog
Z80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples were ...
and Z280; the
Hitachi HD64180 The HD64180 is a Z80-based embedded microprocessor developed by Hitachi with an integrated memory management unit (MMU) and on-chip peripherals. It appeared in 1985. The Hitachi HD64180 "Super Z80" was later licensed to Zilog and sold by them ...
; and the Inmos
Transputer The transputer is a series of pioneering microprocessors from the 1980s, intended for parallel computing. To support this, each transputer had its own integrated memory and serial communication links to exchange data with other transputers. T ...
. The STEbus is designed for 8-bit microprocessors. Processors that normally use a wider data bus (
16-bit 16-bit microcomputers are microcomputers that use 16-bit microprocessors. A 16-bit register can store 216 different values. The range of integer values that can be stored in 16 bits depends on the integer representation used. With the two mo ...
, etc.) can use the STEbus if the processor can handle data in byte-wide chunks, giving the slave as long as it needs to respond. Michael J. Spinks
"Microprocessor System Design: A Practical Introduction"
2013. p. 158, 162, 166.
The STEbus supported processors from the popular Z80, the 6809, to the 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. The 6809 used cycle stretching. 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. Researchers describe STEbus systems as rugged, adaptable, and cost effective.


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. As time went on, PC systems did away with the need for card cages and backplanes by moving to the PC/104 format where boards stack onto each other. While not as well-designed as the STEbus, PC/104 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. As of 2013, some manufacturers still support STEbus, G64, Multibus II, and other legacy bussed systems. The IEEE have withdrawn the standard, not because of any faults but because it is no longer active enough to update.


Physical format

3U Eurocard - The most common size was the 100 x 160 mm Eurocard. 6U Eurocard - Rare, sometimes used in VMEbus hybrid boards


Connector

DIN 41612 DIN 41612 was a DIN standard for electrical connectors that are widely used in rack based electrical systems. Standardisation of the connectors is a pre-requisite for open systems, where users expect components from different suppliers to operat ...
, rows a and c, 0.1" pitch. VME/STE hybrid boards have the STEbus and VMEbus sharing the VME P2 connector, VME signals on row b. For this reason, STEbus boards may not use row b for any purpose.


Pinout

Active low signals indicated by asterisk. GND: Ground reference voltage +5V: Powers most logic. +12V and -12V: Primarily useful for RS232 buffer power. The +12V has been used for programming voltage generators. Both can be used in analogue circuitry, but note that these are primarily power rails for digital circuitry and as such they often have digital noise. Some decoupling or local regulation is recommended for analogue circuitry. VSTBY: Standby voltage. Optional. This line is reserved for carrying a battery backup voltage to boards that supply or consume it. A 3.6V NiCad battery is a common source. The STEbus spec is not rigid about where this should be sourced from. In practice, this means that most boards requiring backup power tend to play safe and have a battery on board, often with a link to allow it to supply or accept power from VSTBY. Hence you can end up with more batteries in your system than you need, and you must then take care that no more than one battery is driving VSTBY. D0...7: Data bus. This is only 8-bits wide, but most I/O or memory-mapped peripherals are byte-oriented. A0...19: Address bus. This allows up to 1 MByte of memory to be addressed. Current technology is such that processor requiring large amounts of memory have this on the processor board, so this is not a great limitation. I/O space is limited to 4K, to simplify I/O address decoding to a practical level. A single 74LS688 on each slave board can decode A11...A4 to locate I/O slave boards at any I/O address with 16-byte alignment. Typically 8 small jumpers or a single unit of 8
DIP switch A DIP switch is a manual electric switch that is packaged with others in a group in a standard dual in-line package (DIP). The term may refer to each individual switch, or to the unit as a whole. This type of switch is designed to be used on a ...
es or two binary-coded hexadecimal
rotary switch A rotary switch is a switch operated by rotation. These are often chosen when more than 2 positions are needed, such as a three-speed fan or a CB radio with multiple frequencies of reception or "channels". A rotary switch consists of a spindl ...
es are used to give each I/O slave board a unique address. CM0...2: Command Modifiers. These indicate the nature of the data transfer cycle. A simple processor board can drive CM2 high for all bus access, drive CM1 from a memory/not_IO signal, and CM0 from a read/not_write signal. CM2 low state is used only during "attention request" phases (for interrupts and/or DMA cycles) for Explicit Response mode. When Implicit Response mode is used, the bus master polls the slave boards to find which one has triggered the Attention Request and reset the signal source. In that case, Vector-fetch is not used. ATNRQ0...7*: Attention Requests. These are reserved for boards to signal for processor attention, a term which covers Interrupts and Direct Memory Access (DMA). The wise choice of signal does not commit these lines to being specific types, such as maskable interrupts, non-maskable interrupts, or DMA. The number of Attention Requests reflects the intended role of the STEbus, in real-time control systems. Eight lines can be priority encoded into three bits, and is a reasonably practical number of lines to handle. BUSRQ0...1* and BUSAK0...1*: Bus Requests and Bus Acknowledge. Optional. Used by multi-master systems. The number of Attention Requests reflects that the STEbus aims to be simple. Single-master systems are the norm, but these signals allow systems to have secondary bus masters if needed. DATSTB*: Data Strobe. This is the primary signal in data transfer cycles. DATACK*: Data Acknowledge. A slave will assert this signal when to acknowledge the safe completion of a data transfer via the STEbus. This allows STEbus systems to use plug-in cards with a wide variety of speeds, an improvement on earlier bus systems that require everything to run at the speed of the slowest device. TFRERR*: Transfer Error. A slave will assert this signal when acknowledging the erroneous completion of a data transfer via the STEbus. ADRSTB*: Address Strobe. This signal indicates the address bus is valid. Originally, this had some practical use in DRAM boards which could start strobing the address lines into DRAM chips before the data bus was ready. The STEbus spec was later firmed up to say that slaves were not allowed to start transfers until DATSTB* was ready, so ADRSTB* has become quite redundant. Nowadays, STEbus masters can simply generate DATSTB* and ADRSTB* from the same logic signal. Slaves simply note when DATSTB* is valid (since the bus definition insists that the address will also be valid at the same time as the data). ADRSTB* also allows a bus master to retain ownership of the bus during indivisible read-modify-write cycles, by remaining active during two DATSTB* pulses. The sequence matches that of the 68008's bus. Other CPUs may require additional logic to create read-modify-write cycles. SYSCLK: System Clock. Fixed at 16 MHz. 50% duty cycle. SYSRST*: System Reset. STE bus information
The backplane connects all the DIN connectors in parallel. So a STEbus
expansion card In computing, an expansion card (also called an expansion board, adapter card, peripheral card or accessory card) is a printed circuit board that can be inserted into an electrical connector, or expansion slot (also referred to as a bus sl ...
sees the same signals no matter which slot of the backplane it is plugged into. Paul Qualtrough
"STEbus-based Hardware for a Model Railway Control System"
1998.


Types of signals

The SYSCLK must be driven by only one board in the system. As explained in the standard, this signal shall be generated by the System Controller. The System Controller is also in charge of the Bus Arbitration in case there are multiple masters. When there is only one Master, the System Controller is not needed, and SYSCLK can be generated by the Master board


Technical notes

* Signal inputs must be Schmitt trigger * Only one TTL load per bus line signal per board * Signal outputs must have a fanout of 20 * Backplane can have up to 21 sockets * 50mm maximum length of bus signal line PCB trace on any board * 500 mm maximum length of bus signal line length * Active bus-termination recommended (270R pull-up to 2.8V) *
7400 series The 7400 series of integrated circuits (ICs) are a popular logic family of transistor–transistor logic (TTL) logic chips. In 1964, Texas Instruments introduced the SN5400 series of logic chips, in a ceramic semiconductor package. A lo ...
chips are often used to build custom control boards, directly connected to the STEbus.


External links


STEbus (IEEE1000) standard
(available to subscribers and IEEE members)

cost: 192 Swiss francs


References

{{DEFAULTSORT:STEBus Computer buses Digital electronics Motherboard