
CD2481
—
Programmable Four-Channel Communications Controller
94
Datasheet
At the end of a non-transparent frame:
At the end of a transparent frame:
Note:
For the transparent Bisync frames (receive only), the first DLE is not included in the CRC
calculation, with the exception of DLE-SYN, in which the SYN is also not included.
Transparency Support
In transmit, the CD2481 generates CRC according to the tables earlier, however the host must
insert the DLE manually in the data stream.
On receive, the CD2481 checks CRC according to the tables earlier, however it does not strip the
DLE from the data stream.
7.6.4
BCC Computation Formulas
In Bisync mode, the CD2481 can use either CRC-16 or LRC. The mode used is determined by the
setting of the LRC bit in COR2[7]. CRC-16 uses the polynomial:
x**16 + x**15 + x**2 + 1
preset to all zeroes. LRC performs a parity check on each bit of each character in the frame in a
longitudinal or
“
vertical
”
manner. For example, in the following three byte frame, the LRC, with
even parity, would be computed as shown:
Character 1:10010001
Character 2:11000001
Character 3:11000001
LRC:
10010001
Data
Data
SYN
Data
Data
SY
N
SY
N
Data
Data
CRC
CRC
CRC
Data
Data
ETX
BCC
BCC
Data
Data
ETB
BCC
BCC
Data
Data
ITB
BCC
BCC
CRC Ends
Data
Data
DLE
ETX
BCC
BCC
Data
Data
DLE
ETB
BCC
BCC
Data
Data
DLE
ITB
BCC
BCC
CRC
CRC