Specifications

Acorn Enhanced Expansion Card Issue 5, August 1994 21
Acorn Enhanced Expansion Card
MEMC expansion card timing
A typical cycle with timing parameters is shown in Figure
21: MEMC expansion card timing on page 20. The
sequence consists of a MEMC expansion card access,
a DRAM refresh by MEMC, with the end of the
expansion card access delayed by one REF8M cycle.
The Acorn enhanced expansion
bus backplane pin-out
The Acorn enhanced expansion bus pin-out is based
around the existing two row pin-out of the basic Acorn
expansion bus. A third row as been added and several
pins which were previously reserved have been used to
create the Acorn enhanced expansion bus pin-out. The
enhanced pin-out includes an extension on the existing
I/O data and address buses, and also includes the extra
control signals needed for the DEBI.
The Acorn enhanced expansion bus backplane pin-out
is shown below. The signals shown in bold show the
enhancements made to the existing Acorn expansion
bus.
1. The A3000 is 5 volt only. Pins 2c and 32c are not con-
nected.
2. On the A3000 External expansion card bus connector,
this signal is MS[0].
Table 2: Acorn enhanced expansion bus backplane pin-out
Pin a b c Description
1 0v 0v 0v ground
2 LA[15] nBW -5v
1
processor BYTE/WORD
3 LA[14] LA[23] 0v ground
4 LA[13] LA[22] 0v ground
5 LA[12] LA[21] Ready ready stretch
6 LA[11] LA[20] MS
2
MEMC exp card select
7 LA[10] LA[19] DRQ
3
DMA request
8 LA[9] LA[18] DACK
3
DMA acknowledge
9 LA[8] LA[17] Reserved
10 LA[7] LA[16] Reserved
11 LA[6] LA[1] TC
3
terminal count
12 LA[5] LA[0] RST reset (see note)
13 LA[4] 0v PR/W read / not write
14 LA[3] BD[31] IOWR write strobe
15 LA[2] BD[30] IORD read strobe
16 BD[15] BD[29] PIRQ normal interrupt
17 BD[14] BD[28] PFIQ fast interrupt
18 BD[13] BD[27] EAS
4
EASI space strobe
19 BD[12] BD[26] I2Cclk I
2
C serial bus clock
20 BD[11] BD[25] I2Cdat I
2
C serial bus data
21 BD[10] BD[24] Reserved
22 BD[9] 0v PS
5
IOC expansion card select
23 BD[8] BD[23] IOGT MEMC card handshake
24 BD[7] BD[22] IORQ MEMC card request
25 BD[6] BD[21] BL I/O data latch control
26 BD[5] BD[20] 0v supply
27 BD[4] BD[19] CLK2 2 MHz synchronous clock
28 BD[3] BD[18] CLK8 8 MHz synchronous clock
29 BD[2] BD[17] REF8M 8 MHz reference clock
30 BD[1] BD[16] +5v supply
31 BD[0] 0v CLK16 16MHz system clock
32 +5v +5v +12v
1
supply
To support interface cards of the existing Acorn
expansion bus type (i.e. DEBI not included) only the
signals on rows a and c are required, therefore the use
of a 64-way DIN 41612ac connector is sufficient to
connect to the expansion bus. If the interface designer
chooses to use the DEBI then they must interface to
rows a, b and c of the backplane, i.e. they must use a
DIN 41612abc type connector.
The reserved pins must be left unused, as in some early
machines these pins carry co-processor signals.
I
2
C bus
This bus is primarily intended for Acorn's use and it
should only be used within the computer.
• The maximum recommended load per expansion
card is 20pF.
• The speed and timings may vary between computer
models and operating systems.
• This bus is not a full implementation of the I
2
C
specification (e.g. it does not support other bus
masters).
• This bus may not be incorporated in future machines.
Reset
The RST signal is the system reset signal. It is driven low
at power-on, or by a user reset. It is an open-collector
signal, and expansion cards may drive it, to generate a
system reset. The pulse width should be at least 50ms.
Power consumption
It is strongly recommended that expansion cards work
from the 5 volt rail only. Acorn expects future models to
continue the trend, started with A3000, towards single
supply computers.
The maximum current drain allowed from the computer
+5 volt supply is 1.0A per expansion card slot, for
external expansion cards. See Appendix A for details of
internal expansion cards.
Each expansion card may draw a maximum of 10mA
from the −5V rail and 250mA from the +12V rail. These
voltages are not available on the A3000 and may not be
available on new computers.
Double width cards may use twice the above current
values when used with Archimedes.
3. Not all machines with DMA will support all slots. For in-
stance, the Risc PC 600 only supports slots 0 and 1.
4. Expansion cards designed to be used in platforms prior
to the Risc PC must leave pin C18 as no-connect. Expan-
sion cards designed for use on both Risc PC and non-Risc
PC machines must possess the ability of making C18
no-connect by the use of a jumper or link etc.
5. On the A3000 External expansion card bus connector
this signal is PS[0].