Specifications

Appendix A Issue 5, August 1994 A-1
Mini Expansion Card Specification
Appendix A: Mini Expansion Card Specification
Introduction
This appendix describes some differences to the text in
the Acorn Enhanced Expansion Card Specification that
apply to mini expansion cards.
The mini expansion card is the name given to the single
internal expansion card upgrades which fit in the A4000
and A3000 series computers. The mini expansion card
interface plugs into four SIL Molex sockets (two 17-way
connectors and two 5-way connectors) towards the back
of the main PCB. It has a metal back panel as provision
for external plug and socket connections.
Some information given in the Acorn Enhanced
Expansion Card Specification is repeated here in the
interests of clarity.
Types of mini expansion card
There are two types of mini expansion card: IOC access
and MEMC access. All mini expansion cards decode at
Podule slot 1.
IOC access mini expansion cards
These use IOC to generate the
IOGT signal required by
MEMC. IOC allows four variations of access timing to I/O,
selected by address value. These different timings are
slow, medium, fast and synchronous. They are defined in
the Acorn Enhanced Expansion Card Specification,
though the following signals are not supplied to the mini
expansion card interface:
• CLK8 (not supplied, though signals shown are relative
to it)
• RBE
• BD[8:15]
• LA[14:15] (the podule slot decodes).
See Figure 14: General timing for slow, medium, and fast
cycle types on page 15 of the Acorn Enhanced
Expansion Card Specification, and note that other figures
refer to these signals, which should be ignored for mini
expansion cards. Also note comments in MEMC mini
expansion card timing below, regarding access timings.
Note that a synchronous read of the byte at base address
0 of the mini expansion card is performed to establish the
expansion card identity.
MEMC access mini expansion cards
These work directly with MEMC IORQ and IOGT signals,
and can therefore optimise the cycle time of the mini
expansion card application. The signals mentioned in the
previous section are obviously not supplied.
Warning: If the MEMC I/O space is accessed when no
MEMC mini expansion card is present, the system will
hang waiting for IOGT to be returned, and will need to be
reset.
Note: MEMC mini expansion cards must also have some
IOC access logic to allow synchronous read of the
expansion card identity byte, initial ROM data, etc.
Notes on expansion card
specification
System architecture
All expansion cards except MEMC podule 1 and IOC
podule 1 are irrelevant with respect to mini expansion
cards.
I/O space memory map
Only the addresses reserved in the table (on page 5 of
the Acorn Enhanced Expansion Card Specification) for
expansion card 1 apply to the mini expansion card.
Data bus mapping
As only BD[0:7] are available for mini expansion cards,
the I/O data bus BD[0:7] connects to the main system
data bus D[0:31] via bi-directional latches as follows:
• During WRITE to peripheral, BD[0:7] is mapped to
D[16:23].
• During READ from peripheral, BD[0:7] is mapped to
D[0:7].
MEMC mini expansion card timing
Note that the timings quoted in the tables of the Acorn
Enhanced Expansion Card Specification are relative to
the indicated vertical timing marks, but the drawn signal
positions are not necessarily correct with reference to the
REF8M clock signal.