Specifications

Acorn Enhanced Expansion Card Issue 5, August 1994 5
Acorn Enhanced Expansion Card
drive the PIRQ line low. Both interrupt lines have a
resistive pullup of 1k2Ω. In order that the MPU can
determine which expansion card is generating the
interrupt, an expansion card which is driving the PIRQ
line low must also set its IRQ status bit high.
An expansion card generating a FIQ interrupt must drive
the PFIQ line low. In order for the operating system to
determine which expansion card is generating the
interrupt, a card which is driving the PFIQ line low must
also set its FIQ status bit high. (See Expansion card
identity low byte on page 6.)
Some variants of the computer (Archimedes 400/1, 540,
A5000 and R-Series) have extra logic on the backplane
PCB, for expansion card interrupt management. The
default/power on state of the logic leaves expansion
card interrupts enabled, i.e. the logic can be ignored and
the system will behave identically to the A300 and early
A440 models. Two functions are added by the extra
logic, a mask register and a status register. The logic is
fitted to support RISC iX.
Expansion card interrupt mask register
This section only applies to those variants of the
computer that have extra logic on the backplane PCB,
for expansion card interrupt management (Archimedes
400/1, 540, A5000 and R-Series).
This register allows individual expansion card
IRQ
interrupts to be masked off, and provides a means of
implementing an interrupt priority scheme for expansion
cards.
Writing a ‘0’ to a bit in the expansion card interrupt mask
register disables interrupts from the corresponding slot
on the backplane.
Writing a ‘1’ to a bit in the expansion card interrupt mask
register enables interrupts from the corresponding slot
on the backplane.
The mechanism for identifying which slot is generating
the interrupt is described in the section entitled
Expansion card interrupt status register below.
The mechanism for clearing the interrupt from a
particular slot will depend on the device installed in that
slot.
The table below shows the correspondence between bit
position and slot number.
(RSVD = Reserved)
Expansion card interrupt status
register
This read-only register allows the processor to identify
BD7 BD6 BD5 BD2 BD1 BD0BD4 BD3
RSVD RSVD RSVD RSVD slot 3 slot 2 slot 1 slot 0
which expansion card is generating the interrupt without
scanning the IRQ flag on each expansion card.
A logic one read in a bit position indicates that the slot is
enabled and interrupting. The status register uses the
same bit allocation as the mask register.
Layout and drive
The drive capability of the expansion card interface is
limited, so expansion cards should offer the lowest
practical load. Expansion card PCB track layout should
minimise track length to the DIN edge connector, to
avoid ringing or excessive capacitive loading of the
interface signals.
Track lengths from the DIN41612 connector should
always be less than 50mm with the REF8M tracking less
than 25mm.
Signals driving the expansion card interface should
include series damping resistors if possible, to reduce
ringing and ground bounce problems in the card and the
computer. About 68Ω is usually sufficient.
Data signals BD[0:31] should have a maximum logic low
input current of -0.4mA, e.g. one LS TTL gate input load.
The recommended drive capability for drivers of the data
bus is 6mA (e.g. an HC series bus driver). The use of HC
or AC family logic is recommended.
Address lines should have a maximum logic low input
current of -1.2mA; again the use of HC or AC logic is
recommended.
Control signals: the logic low input current of control
signals, e.g. CLK2, CLK8 and REF8M should be less
than -0.4mA (e.g. one LS TTL gate input load, input
load = 20pF max). Again the use of HC or AC logic is
recommended.
Open drain/open collector drivers should be able to sink
at least 6mA (1k2Ω pull up, plus four LS TTL gate input
loads) and still achieve a logic low voltage of less than
0.5V.
All output signals from the computer to the expansion
card interface are TTL logic compatible. Expansion
cards may drive the interface with TTL levels, but CMOS
logic levels are recommended. The signals Ready, RST,
PIRQ, PFIQ, IOGT and BL are open drain/collector with
a resistive pull-up.
In the case of the A3000 mini expansion card interface,
it is recommended that the load on each signal does not
exceed 3 HCT gates. It is also recommended that the
drivers provided on the data bus be capable of driving at
least 7 HCT and 3 TTL loads.
Heating
The power allocated for an external expansion card (for
example, the A3000) should not be dissipated within the
case even if no external card is fitted. This extra heating
may create excessive temperatures within the case.
If an Archimedes is fitted with a 4-way backplane, a fan