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author AJR <ajrhacker@users.noreply.github.com>2021-01-22 11:12:38 -0500
committer AJR <ajrhacker@users.noreply.github.com>2021-01-22 11:12:38 -0500
commit32f64dfce658fea5f3d8b9247055976b75020098 (patch)
tree704c52162b6d8341fe15913bb13e21699e9db2b5
parentb3daa829c28f836d7e687ff6f4bf0c49048650f1 (diff)
mac128.cpp: VIA note
-rw-r--r--src/mame/drivers/mac128.cpp18
1 files changed, 17 insertions, 1 deletions
diff --git a/src/mame/drivers/mac128.cpp b/src/mame/drivers/mac128.cpp
index 96678472ace..5023a2cb587 100644
--- a/src/mame/drivers/mac128.cpp
+++ b/src/mame/drivers/mac128.cpp
@@ -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.
****************************************************************************/