Specifications
Apple IIGS
#68: Tips for I/O Expansion Slot Card Design 1 of 6
Apple II
Technical Notes
Developer Technical Support
Apple IIGS
#68: Tips for I/O Expansion Slot Card Design
Written by: Rob Moore & Jim Luther September 1989
This Technical Note points out several potential problem areas developers should know about
when designing I/O expansion slot cards for the Apple IIGS.
This Note is written for experienced design engineers. It is not intended to be a tutorial on Apple
IIGS I/O expansion card design techniques, but rather to point out possible problem areas and
pitfalls to help developers produce successful and reliable expansion cards.
The 65C816 PH2 Clock versus the Expansion Slot PH0 Clock
It is important to understand the timing of the 65C816 Phase 2 clock (PH2) on the IIGS, because
several of the expansion slot signals are actually related to the PH2 clock timing, rather than the
1 MHz Phase 0 clock (PH0) available at the expansion slots. Unlike the Apple IIe, the PH2
clock at the CPU is not the same as the PH0 clock found at the expansion slots. The PH2 clock
runs at a variety of periods, depending on whether the CPU is doing a normal 350 nanosecond
2.8 MHz cycle, a extended 700 nanosecond RAM refresh cycle, an isolated slow cycle, or
consecutive 980 nanosecond 1.024 MHz slow cycles. During isolated slow cycles, or the first of
a series of consecutive slow cycles, the fast side of the system must wait to synchronize with the
1 MHz side of the system. This synchronization results in an average cycle time of about 1.5
microseconds.
Cycle Type Low High Period
Normal 2.8-MHz cycle 140ns 210ns 350ns
Refresh extended cycle 140ns 560ns 700ns
Isolated 1-MHz cycle 140ns typ. 1.33µs avg. 1.5µs
Consecutive 1-MHz cycles 140ns 840(980)ns 980ns
Table 1–PH2 Clock Times
The Mega II Select Signal
On the Apple IIGS, the Mega II select signal (/M2SEL) is used as the enable to the slower, 1
MHz side of the system. It goes active (low) whenever the 1 MHz side RAM or I/O areas are
accessed. Accesses that cause /M2SEL to be asserted include shadowed video writes, any