Zenith Z-100 Series, Technical Manual

Page: 270 / 493

Page 4.60
CIRCUIT DESCRIPTION
Video logic boards can have either 32K or 64K parts installed.
Current software, however, requires only 32K parts.
CRTRAMSEL asserts whenever pin 11, 10, or 9 asserts. This
connects to U372 pin 12 in the lower left corner of the schema-
tic. It i s c o mbined with VIDBDSEL and DBIN to a s sert
RDBFR ENBL. This line enables U223 during a memory read
operation.
C RTRAMSEL is d ouble-inverted at U 366 pi n 5 t o f o r m
CRTRAMSEL1 at P305 pin 61. This line asserts PHANTOM
*
at U194 pin 4 on MB2. If an S-100 memory card is occupying
the same memory space as VRAM, PHANTOM
* prevents the
CPU from writing to the S-100 memory when it is accessing
video RAM. This permits you to install read/write memory in
the same address space as VRAM without them interferring
with each other.
CRTRAMSEL also goes to U372 pin 3, VIDRAMRDY, through
an inverter. If the CRT-C is busy processing a video signal,
it will not let the CPU access the RAM circuits. U372 pin 2
is also high, causing VIDRAMRDY to go low. This drives RDY
low at U194 pin 12, putting the CPU into a wait state. The
CPU will hold CRTRAMSEL asserted until the CRT-C gives
the CPU control of the video circuits.
Finally, CRTRAMSEL goes to U379 pin 11, part of the CPU/
video arbitration circuits. These circuits synchronize the video
circuits to the CPU circuits and determine when the CPU can
access the video RAM. See the previous paragraph and the
description of the control and timing circuits.