User Manual

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07.93 5

CDDL Protocol - Data link level

4.1.1 CRC

The CRC is composed by two bytes and it is computed using the CRC-2 polynomial

including all the characters: starting from the start frame sequence, included (DLE +

SOP + DTPL), to the whole packet length (DTP).

The CRC-2 algorithm performs a XOR of all the characters including the starting

frame sequence. The algorithm result is inserted in the message putting the most sig-

nificant byte in front of the less significant byte.

4.1.2 Control bytes masking

To avoid that bit masks equivalent to protocol control characters possibly present in

the DTPL, DTP or CRC fields could generate malfunctioning, the CDDL protocol uses

a byte-stuffing technique. The control characters to be masked and the masking se-

quences are as follows:

Control character Masking sequence

EOP DLE, NUL

DLE DLE, DLE

The masking algorithm applies to the complete frame, with the exclusion of start and

end frame sequences [DLE + SOP, DLE + EOP].

The maximum overall length of an on-line frame is 1036 characters (512 x 2 DTP

body packet characters + 2 x 2 DTPL header characters + 2 x 2 checksum characters

+ 2 start frame characters + 2 end frame characters).

4.2 Polling

The master station issues at regular intervals the polling sequence [DLE, ENQ].

The [DLE, ENQ] sequence reception enables the slave station to transmit.

Fig.: 2

The slave station answers anyway to the selection sending a packet. If no data to be

transmitted are available, the packet will be empty (empty DTF).

The polling sequence is shown in Fig.: 2