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CDDL Protocol - Data link level

4 CDDL Protocol - Data link level

The CDDL protocol at the data link level belongs to the "polling-selecting" type, with

"stop-and-wait" transmission and positive acknowledgement. This protocol needs a

master station. Usually the master is the Gateway, that starts the data transmission

(polling) or asks the permission to transmit (calling) to the slave station, usually a sub-

system. The slave station is normally in a receiving status and it transmits only if it is

requested to do so.

4.1 Transmitted data format

The data transmitted among the stations are collected in frames. A frame has the

structure shown in Fig.: 1

The frame is composed by:

• start frame sequence [DLE, SOP] (start of packet)

• frame header DTPL

• packet body DTP (Data Transmission packet)

• CRC-2 checksum (Cycling Redundancy Check 2nd order)

• end frame sequence [DLE, EOP] (end of packet)

The DTPL header (Data Transmission Package Length) is composed by two bytes

containing the packet body DTP length in characters. The byte 0 is the most signifi-

cant, while the byte 1 is the less significant. The length includes the packet body

stuffing, when used (refer to par. 4.1.2 for a description of the byte stuffing technique).

The maximum theoretical value foreseen for the DTPL (DTP length) is 512 charac-

ters, but memory reasons reduce the practical limit to 128 byte. The DTP is composed

by the sequence of bytes to be transmitted.

DLE SOP DTPL DTP CRC DLE EQP

Frame start

sequence

Frame header

Data transmission packet lenght

Packet body

Cyclic redundancy

check CRC - 2

Frame end

sequency

The byte - stuffing algorithm applies

to this

art of the messa

Fig.: 1