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8XC251SA, SB, SP, SQ USER’S MANUAL
13-22
13.8.3 Example 3: RD1:0 = 01, 17-bit Bus, External RAM
In this example, an 87C251SB/83C251SB operates in nonpage mode with a 17-bit external ad-
dress bus interfaced to 128 Kbytes of external RAM (Figure 13-21). The 87C251SB/83C251SB
is configured so that RD# functions as A16, and PSEN# is asserted for all reads. Figure 13-22
shows how the external RAM is addressed in the internal address space.
Figure 13-21. Bus Diagram for Example 3: 87C251SB/83C251SB in Nonpage Mode
PSEN#
A4147-02
WR#
D7:0
A15:8
A7:0
A16
OE#
WE#
A15:8
A16
P2
P0
A7:0
83C251SB
87C251SB
RAM
(128 Kbytes)
CE#
EA#
V
CC
AD7:0
Latch
A16
Data