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74F402
Unit Loading/Fan Out
Note 1: Open Collector
Functional Description
The 74F402 Serial Data Polynomial Generator/Checker is
an expandable 16-bit programmable device which oper-
ates on serial data streams and provides a means of
detecting transmission errors. Cyclic encoding and decod-
ing schemes for error detection are based on polynomial
manipulation in modulo arithmetic. For encoding, the data
stream (message polynomial) is divided by a selected poly-
nomial. This division results in a remainder (or residue)
which is appended to the message as check bits. For error
checking, the bit stream containing both data and check
bits is divided by the same selected polynomial. If there are
no detectable errors, this division results in a zero remain-
der. Although it is possible to choose many generating
polynomials of a given degree, standards exist that specify
a small number of useful polynomials. The 74F402 imple-
ments the polynomials listed in Table 1 by applying the
appropriate logic levels to the select pins S
0
, S
1
, S
2
and S
3
.
The 74F402 consists of a 16-bit register, a Read Only
Memory (ROM) and associated control circuitry as shown
in the Block Diagram. The polynomial control code pre-
sented at inputs S
0
, S
1
, S
2
and S
3
is decoded by the ROM,
selecting the desired polynomial or part of a polynomial by
establishing shift mode operation on the register with
Exclusive OR (XOR) gates at appropriate inputs. To gener-
ate the check bits, the data stream is entered via the Data
Inputs (D), using the LOW-to-HIGH transition of the Clock
Input (CP). This data is gated with the most significant
Register Output (RO) via the Register Feedback Input
(RFB), and controls the XOR gates. The Check Word Gen-
erate (CWG) must be held HIGH while the data is being
entered. After the last data bit is entered, the CWG is
brought LOW and the check bits are shifted out of the reg-
ister(s) and appended to the data bits (no external gating is
needed).
To check an incoming message for errors, both the data
and check bits are entered through the D Input with the
CWG Input held HIGH. The Error Output becomes valid
after the last check bit has been entered into the ’F402 by a
LOW-to-HIGH transition of CP, with the exception of the
Ethernet polynomial (see Applications paragraph). If no
detectable errors have occurred during the data transmis-
sion, the resultant internal register bits are all LOW and the
Error Output (ER
) is HIGH. If a detectable error has
occurred, ER
is LOW. ER remains valid until the next LOW-
to-HIGH transition of CP or until the device has been pre-
set or reset.
A HIGH on the Master Reset Input (MR) asynchronously
clears the entire register. A LOW on the Preset Input (P
)
asynchronously sets the entire register with the exception
of:
1. The Ethernet residue selection, in which the registers
containing the non-zero residue are cleared;
2. The 56th order polynomial, in which the 8 least signifi-
cant register bits of the least significant device are
cleared; and,
3. Register S = 0, in which all bits are cleared.
Pin Names Description
U.L.
Input I
IH
/I
IL
HIGH/LOW
Output I
OH
/I
OL
S
0
–S
3
Polynomial Select Inputs 1.0/0.67 20 µA/−0.4 mA
CWG Check Word Generate Input 1.0/0.67 20 µA/−0.4 mA
D/CW Serial Data/Check Word 285(100)/13.3(6.7) −5.7 mA(−2 mA)/8 mA (4 mA)
D Data Input 1.0/0.67 20 µA/−0.4 mA
ER
Error Output (Note 1) /26.7(13.3) (Note 1) /16 mA (8 mA)
RO Register Output 285(100)/13.3(6.7) −5.7 mA(−2 mA)/8 mA (4 mA)
CP Clock Pulse 1.0/0.67 20 µA/−0.4 mA
SEI Serial Expansion Input 1.0/0.67 20 µA/−0.4 mA
RFB Register Feedback 1.0/0.67 20 µA/−0.4 mA
MR Master Reset 1.0/0.67 20 µA/−0.4 mA
P
Preset 1.0/0.67 20 µA/−0.4 mA
