British Telecommunications Engineering Volume 6 Part 4 page 284

68000 STEbus Processor

A new high-performance CPU card from British Telecom Microprocessor Systems offers designers exceptional processing power on an extremely cost-effective bus — STE. Prior to this introduction, users would almost certainly have had to purchase such a CPU implemented on an expensive high-performance bus like VME.

The board, designated the 4020 , is based around a full 16 bit 68000 running at 8 MHz without wait states. To maximise this device’s computational power, 16 bit implementation is preserved right across the card. The fact that STEbus offers only an 8 bit data path has minimal impact on overall system throughput, because the module has enough memory to allow the CPU to operate locally in most applications.

Up to half a megabyte of RAM, or ROM, or a mix of memory types, can be fitted on board. Moreover, the PWB is designed to accommodate future device technology: its three pairs of JEDEC 32-pin byte-wide memory sites are fully word and byte addressable, which allows a potential 6 Mbyte of memory to be fitted (when the devices are available). A software-controlled memory overlay facility is also provided: once the system boot-up sequence has been completed, the ROM can be switched out of the memory map in favour of RAM, in order to suit differing operating system requirements.

The board is designed for systems requiring extensive processing power, and incorporates arbitration circuitry that allows it to operate in multiprocessor STEbus environments. A software locking feature gives designers the freedom to retain sole use of the bus during time-critical data transfer operations. However, the 4020 will also operate as the sole CPU in an STEbus system. The board also includes logic for handling all of STEbus’ attention request lines and the transfer error signal, plus system clock and counter-timer facilities.

During system development, an on-card RS232 port can be used as a terminal driver, and a basic start-up monitor is included in ROM. In the target system, this port can be redeployed as a simple serial I/O channel: it is brought to the front edge of the card on a 10-way shrouded header, and when connected via ribbon cable to a 9-pin male ‘D’ type connector, provides compatibility with the serial port on an IBM AT.

The 4020 card, which forms part of BT’s Martello range of STEbus systems and peripherals, sets a new standard in STE price and performance. Designed for use with any modern multi-tasking operating system — including OS/9 and Tripos — the card brings the computational power of true 16 bit processing to STE systems. A flexible approach to memory expansion, coupled with the inherent cost advantages of the STE standard, means that high-performance future-proof systems can now be cost-effectively realised.