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mac128.cpp: VIA note

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AJR 2021-01-22 11:12:38 -05:00
parent b3daa829c2
commit 32f64dfce6
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src/mame/drivers/mac128.cpp
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@ -52,7 +52,7 @@ Control B:
02 select WR2
00 int vector = 0
03 select WR3
c0 8 data bits, Rx disabled
oc0 8 data bits, Rx disabled
0f select WR15, external interrupt status/control
08 enable 08
00 select WR0
@ -80,6 +80,22 @@ c0 8 data bits, Rx disabled
09 select WR9
0a enable MIE / NV
VIA notes:
Original 128K Macs use Synertek SYP6522 VIAs. Synertek's AN5 (March 1982)
claims that their VIAs have a difference in shift register behavior
compared to those from other manufacturers. However, this particular
difference is not relevant to the Mac hardware, and later 512K Macs
switched away from SYP6522 (Apple part number 338-6522-A) to either
Rockwell R6522AP (R6522-66) or VTI VL6522-02PC. R6522AP is the most
common VIA on the Mac Plus.
Apple continued sourcing VIAs from the same two manufacturers for the
Mac SE, though their ADB-era VIAs are newer CMOS models with a fix for the
shift register's CB1 input synchronizer which Apple may have specifically
requested. The new Rockwell version (R65NC22) is only labeled with Apple
part number 338-6523 (later Macs use a PLCC version which Apple numbered
338S6523), but VLSI Technology's VL65C22V-02PC is not so disguised.
****************************************************************************/