Specifications

Acorn Enhanced Expansion Card
Acorn Enhanced Expansion Card Issue 5, August 1994 1
Acorn Enhanced Expansion Card
Introduction
This document outlines the expansion card interface
implemented on current Acorn Archimedes, R-series,
BBC A3000 computer systems, and the Acorn Risc PC.
The majority of this document is also relevant to A4000
and the later A3000 series machines that can be fitted
with internal expansion cards (known as mini expansion
cards) – see Appendix A. For more information on the
RISC OS software interface with expansion cards, and
how to write loaders for them, see Expansion cards and
Extension ROMs in Part 13 of the RISC OS 3
Programmer’s Reference Manual, and Part 18 –
Expansion Card Support, of the RISC OS 3
Programmer’s Reference Manual, Vol. 5 (version 3.5
supplement).
This document replaces the Acorn expansion card
specification (Part No 0472,200 Issue 4).
The existing Acorn expansion bus is fully supported
within the enhanced version. The Acorn enhanced
expansion bus has several improvements on its
predecessor; namely, the inclusion of the DMA Extended
Bus Interface, or DEBI for short.
DEBI
DEBI includes DMA (Direct Memory Access) interface
support for expansion cards as well as an Extended
Address Space Interface (EASI). In this documentation,
DEBI is sometimes referred to as ‘the enhancements’. A
breakdown of the Acorn enhanced expansion bus is
shown below.
The Acorn enhanced expansion bus has two parts, the
structure of which is as follows:
1DEBI (enhancements)
2The existing Acorn Expansion bus (for backwards
compatibility)
Figure 1: Breakdown of the Acorn enhanced
expansion bus
DEBI is currently fully implemented on the Risc PC
platform and may be supported in whole or part on future
Acorn products.
DEBI is not supported on platforms before the Risc PC,
and a concise table showing the different interfaces
available on Acorn's range of machines is included in the
section entitled Acorn machine range I/O type inclusions
on page 2.
Acorn enhanced expansion bus
DEBI (enhancements)
Existing Acorn expansion bus
(MEMC expansion cards,
IOC expansion cards, etc.)
See Appendix B of this document for further detailed
information relating to the DEBI.
Acorn expansion bus
The inclusion of the existing Acorn Expansion Bus
interface provides support for all types of existing I/O
hardware. These are known more specifically as IOC
expansion cards, MEMC expansion cards and mini
expansion cards.
Expansion cards for Archimedes computers were
formerly known within Acorn as podules, and some relics
of this nomenclature persist in software (and hence in
some diagrams and software descriptions).
It is important to realise that future systems may have a
different implementation of the I/O system, and in
particular the addresses (and number) of expansion card
locations may change. For this reason, and to ensure that
any device may be plugged into any slot, all driver code
for expansion cards must be relocatable. References to
the direct expansion card addresses should never be
used. It is up to the machine operating system, in
conjunction with the expansion card ID, to determine the
address at which an expansion card should be accessed.
Some models of computer, such as the A400/1 series,
also support a co-processor card on expansion card slot
2. The co-processor interface is a superset of the
expansion card interface. It is for Acorn use only, and is
not described in this document.
It is assumed that the reader is familiar with the basic
concepts of computer systems built around the ARM
micro-processor unit (MPU) and its support devices,
MEMC, VIDC, IOC and IOMD.
On the Risc PC platform, the IOC and MEMC type chips
do not exist as separate devices. Instead, the I/O and
memory functions are combined and performed by a
single chip called the IOMD (Input Output Memory
Device). Thus, references to IOMD in this document refer
to the Risc PC platform, and references to IOC and
MEMC refer to older platforms.
Detailed specifications of the original ARM chip set can
be found in the Prentice-Hall publication: Acorn RISC
Machine Family Data Book, ISBN 0-13-781618-9.
In this document logic active low signals are indicated by
a bar over the signal name, e.g. BL.
Physical dimensions
The expansion card printed circuit board (PCB)
mechanics follow the single Eurocard format. On some
platforms, double (233.4mm) width cards of standard
length (160mm) may be used. All cards have a 25.4mm
high (5 HP) metal back panel, for mounting externally
accessible connectors.
Expansion cards for use inside the Risc PC must be of