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74FR900SSC View Datasheet(PDF) - Fairchild Semiconductor

Part Name
Description
Manufacturer
74FR900SSC
Fairchild
Fairchild Semiconductor Fairchild
74FR900SSC Datasheet PDF : 6 Pages
1 2 3 4 5 6
Functional Description
The 74FR900 allows 9-bit data to be transferred from any
of three 9-bit I/O ports to either of the two remaining I/O
ports. The device employs latches in all paths for either
transparent or synchronous operation. Readback capability
from any port to itself is also possible.
Data transfer within the 74FR900 is controlled through use
of the select (S0 and S1) and output-enable (OEA, OEB and
OEC) inputs as described in Table 1. Additional control is
available by use of the latch-enable inputs (LEAC, LECA,
LEBC, LECB) allowing either synchronous or transparent
transfers (see Table 2). Table 1 indicates several readback
conditions. By latching data on a given port and initiating
the readback control configuration, previous data may be
read for system verification or diagnostics. This mode may
be useful in implementing system diagnostics.
Data at the port to be readback must be latched prior to
enabling the outputs on that port. If this is not done, a
closed data loop will result causing possible data integrity
problems. Note that the A and B ports allow readback with-
out affecting any other port. Port C, however, requires inter-
ruption of either port A or B to complete its readback path.
PINV controls inversion of the C8 bit. A low on PINV allows
C8 data to pass unaltered. A high causes inversion of the
data. See Table 3. This feature allows forcing of parity
errors for use in system diagnostics. This is particularly
helpful in 486 processor designs as the 486 does not pro-
vide odd/even parity selection internally.
TABLE 1. Datapath Control
Inputs
S0 S1 OEA OEB OEC
Function
L
X
H
L
L Port A to Port C
L
L
H
H H Port A to Port B
L O H H L Port A to B+C
H
L
L
L
H Port B to Port A
H
X
H
L
L Port B to Port C
HO
L
L
L Port B to A+C
X
H
L
L
H Port C to Port A
X H H H H Port C to Port B
X
H
L
H
H Port C to A+B
X X H L H Outputs Disabled
L
L
L
X
X (Readback to A)
(Note 1)
L H L X L (Readback to A or C)
(Note 1)
H L X H X (Readback to B)
(Note 1)
H H X H L (Readback to B or C)
(Note 1)
Note 1: Readback operation in latched mode only. Transparent operation
could result in unpredictable results.
TABLE 2. Latch-Enable Control
LExx
L
L
H
Input
L
H
X
Output
L
H
Q0
L = LOW Voltage
H = HIGH Voltage Level
TABLE 3. PINV Control
PINV
C8
L
L
L
H
H
L
H
H
A8 or B8
L
H
H
L
Q0 = Output state prior to LExx LOW-to-HIGH transition
Logic Diagram
www.fairchildsemi.com
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