DEC VT220 Technical Manual page 101

6.2.4.1 Access Mux
outputs based on select
logic. Video logic inputs
when select is high.
The access
input (EN
when select
mux (Figure 6-13) selects
CHR GEN RW H) from the CPU
is low and CPU logic inputs
The video logic accesses character generator circuits to load
character and attribute values during DMA transactions, and to
process those values out. The CPU logic accesses the character
generator to load alternative character formats into the character
output circuit, or to read back previously loaded characters.
Later in this
Figure 6-13.
chapter, Table 6-2 describes the signals shown in
FROM
ATTRIBUTE
CIRCUITS
FROM
TIMING
GEN
FROM
9007
FROM
il",r
CPU
lOGIC
(Al T SEL A TTRIBU TE)
4A
AlT SEL
4B
4Y
3A
1
lBA elK l
3B
3Y
T5
L
ACCESS
VIDEO
2A
MUX
LATCH H
..---
28
2Y
Vl_T L
11'
lBA ClR H
r
1
B
1
Y
ALTSElH
SEl
L8A ClKS H
CHR ADDR WR l
I
lBA ClRS H
ENCHRGEN RW H
TO
CHAR
OUTPUT
}
TO
LINE
BUFFER
Figure 6-13 Access Mux Block Diagram
6-17