Specifications

Acorn Enhanced Expansion Card Issue 5, August 1994 3
Acorn Enhanced Expansion Card
Note: ✽ denotes an I/O interface which is resident
outside of the machine, rather than inside.
System architecture
The number of expansion card slots varies with
computer model:
• A305/310 have an optional 2-slot backplane.
• A440 and A400/1 series have a 4-slot backplane.
• A3000 has one internal mini expansion card and one
external expansion card connector.
• A540 has a 4-slot backplane, one occupied by a SCSI
card.
• A5000 has a 4-slot backplane, optional on some
models.
• A3010, A3020 and A4000 have one mini expansion
card connector.
• Risc PC has a 2 or 4-slot backplane. Third parties
may supply 6- and 8-slot backplanes.
Expansion card access speed
IOC expansion cards
IOC expansion cards are mapped through the IOC, and
may be accessed at one of four different cycle speeds,
as determined by the address at which they are
selected. The four cycle types are designated slow,
medium, fast and synchronous. Their timings are
detailed in the section entitled IOC expansion cards on
page 10.
MEMC expansion cards
The cycle timing of a MEMC expansion card access
must be controlled by the expansion card itself. A simple
state machine clocked by the 8 MHz reference signal
(REF8M) can be used to control these cycles. Refer to
the section entitled MEMC expansion cards on page 18.
EASI expansion cards
EASI expansion cards are mapped through IOMD, and
may be accessed at one of two different cycle speeds,
as determined by the contents of the Expansion Card
Timing Control Register (ECTCR) located in IOMD
address space.
R140
● ●
R260
● ●
A540
● ●
A3000
✽ ✽ ●
A3010
●
A3020
●
A4000
●
A5000
● ●
Risc PC
● ● ● ●
Table 1: I/O interfaces used on the Acorn product range
Machine
MEMC
controlled
IOC
controlled
Mini
Expansion
DEBI
DMA EASI
The two cycle types are designated A and C. Their
timings are detailed in Appendix B in Extended address
space (EASI) timings on page B-3.
DEBI expansion cards
DEBI expansion cards are controlled by registers inside
IOMD. Each DMA channel has two buffers, which can be
separately controlled, to transfer data from different
areas of memory. These provide a ‘double buffering’
system for efficient data transfer. The DMA channel has
a control and a status register. Each buffer is separately
controlled by a pair of counter registers called the
current and end registers. These two registers are used
by IOMD to keep a track of the position of the DMA
transfer within main memory.
The four cycle types available for each DMA channel are
set by the DMA Cycle Timing Control Register
(DMATCR). The registers mentioned in this section and
all DMA timing information is covered in more detail in
Appendix B.
Expansion card size
MEMC and IOC expansion cards
The expansion bus can be either 8 or 16 bits wide (or, on
the Risc PC, 32 bits), allowing both byte and half-word
access. Each expansion card slot has 4096 word
addresses (for example, 03xx4000, 03xx4004,
03xx4008 etc.). The address range covered by each of
the five different address modes is identical
(synchronous, fast, medium, slow and MEMC). At each
word address, because of the data width restriction, it is
only possible to access either a byte or a half-word.
Therefore, the size of an expansion card which is only
byte-wide is 4KB, and the size of a half-word expansion
card is 8KB. Some of the space will be used by the
expansion card identification, the exact amount being
chosen by the designer. Refer to Expansion card identity
on page 6.
DEBI and EASI expansion cards
The EASI bus can be 8, 16 or 32 bits wide, allowing byte,
half-word and word access. Each expansion card slot
has 16 million byte wide addresses or alternatively 4
million word wide addresses (total 16 MB per EASI slot).
It is important to note that when the ARM CPU does a
byte read or write, the expansion card hardware must
use the bottom 2 address lines to steer the byte to or
from the correct position in the 32 bit word. The CPU can
only read/write in either byte or word wide modes, and
therefore half-word transfers to the expansion cards are
treated as words. The EASI space used as word-wide is
16MB but for half-word wide it becomes 8MB. Some of
the space can be used by the expansion card
identification, the exact amount being chosen by the
designer.
The DMA can use byte, half-word or word wide data
transfers at four different transfer cycle speeds. The four
cycle types/speeds which can be used are type A, B, C