Specifications

2 Issue 5, August 1994 Acorn Enhanced Expansion Card
Acorn Enhanced Expansion Card
the single Eurocard dimension only – double Eurocards
will not fit. All expansion cards fitted to the Risc PC
should include an EMC gasket where required, fitted
inside the rear panel. The EMC gasket should be as
detailed in the section entitled EMC design on page 22
and as shown in Acorn Drawing No 0297,093.
Each expansion card can be fitted with a 64 or 96 way
DIN41612 type connector for interface to the expansion
card slot.
IOC and MEMC expansion cards will have a 64 way
connector with rows a and c fully loaded. The new DEBI
expansion cards must be fitted with a 96 way connector
with rows a, b and c fully loaded. Pin out differences
between DEBI and the existing Acorn expansion card
are shown in more detail in the section entitled The
Acorn enhanced expansion bus backplane pin-out on
page 21.
A double-width expansion card, when viewed from
above, with the metal back panel towards you, should
have the DIN 41612 connector fitted in the left hand
position.
Further details are given in the Mechanical specification
on page 23.
Types of expansion card
All I/O is memory-mapped. The varying techniques of
interfacing to the Acorn enhanced expansion bus give a
wide variety of access speeds as well as a wide variety
of data transfer widths and sizes. There are four different
types of interfaces included. These are:
• The IOC expansion card type (the IOC generates all
control signals to the expansion card).
• The MEMC expansion card type (the MEMC
IOGT
and IORQ signals are interfaced directly by the
expansion card). Note: On the R260, A540 and
A5000, MEMC’s IORQ and IOGT signals are actually
passed through synchronising logic before becoming
the expansion bus IORQ and IOGT signals.
• The DMA controlled type (IOMD controlled interface –
part of the DEBI spec).
• The Extended Address Space Interface (EASI)
(IOMD controlled interface – part of the DEBI spec).
On the Risc PC, the IOC and MEMC devices have been
replaced with a single device called the IOMD. All
signals that once went between MEMC and IOC are now
internal to IOMD on the Risc PC. The IOMD controlled
interface creates all control signals so that they appear
on the backplane as if MEMC and IOC still exist.
IOC expansion cards
For IOC expansion cards, IOC controls the I/O cycle by
returning IOGT to MEMC. IOC offers four different
access timings, selected by address value. All
expansion cards must have either IOC expansion card
logic, or EASI type interface capabilities, for the
expansion card ID system.
MEMC expansion cards
With MEMC access cycles the expansion card works
directly with the MEMC
IORQ and IOGT signals. This
means the expansion card designer can create
optimised cycle timings for his or her application. System
software should not access MEMC space unless it is
certain that an expansion card is present to return IOGT.
If IOGT is not returned, the I/O system remains hung up
waiting for it. While in the hung state the MPU clocks are
stopped and only the video display process can
continue. After 10 microseconds – for ARM2 – the MPU
register state cannot be relied upon. The computer must
be restarted with a system reset. The ARM610,
however, employs static type registers, whose contents
can theoretically be relied upon indefinitely, although a
system reset will still be needed to restart it.
EASI expansion cards
The Extended Address Space Interface (EASI) is part of
the DEBI specification. Transfer of data can be 32, 16
and 8 bits wide. The EASI bus supports two different
cycle types – as described in Appendix B: DMA
Extended Bus Interface. The EASI can be used for the
transfer of the expansion card ID.
The title EASI expansion card should only be given to an
expansion card which includes any form of the EASI, but
not DMA. If an expansion card includes DMA support as
well as an EASI capability it should be termed a DEBI
expansion card.
Note: EASI expansion cards are not supported on
platforms before the Risc PC.
DEBI expansion cards
The DMA Extended Bus Interface supports 32, 16 or 8
bit wide direct memory access. One of four cycle types
is used to transfer data to and from the expansion bus.
These cycle types vary in length and are named A, B, C
and D, with A being the longest and D the shortest. The
DEBI therefore provides a wide range of access cycle
types to cover a range of peripheral speeds.
Expansion cards containing support for DMA should be
termed ‘DEBI’ expansion cards.
Note: DEBI expansion cards are not supported on
platforms before the Risc PC.
Acorn machine range I/O type
inclusions
The table below provides a quick look-up detail of the
different I/O interfaces used on the Acorn product range:
Table 1: I/O interfaces used on the Acorn product range
Machine
MEMC
controlled
IOC
controlled
Mini
Expansion
DEBI
DMA EASI
A305
● ●
A310
● ●
A420
● ●
A440
● ●
A440/1
● ●