Physical layer

ST7570

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Doc ID 17526 Rev 2

9.3

Frame structure at physical level

The frame at physical level is compliant with the IEC61334-5-1 and is composed of 45 bytes

(360 bits) as follows:

●

2 byte preamble (PRE) (AAAAh);

●

2 byte start subframe delimiter (SSD) (54C7h);

●

38 byte physical service data unit (P_sdu);

●

3 byte for pause or alarm;

The bytes are sent from the most significant byte (MSB) to the least significant byte (LSB).

Bits within the byte are packed with the same order (msb to lsb).

Figure 14.

Physical frame format

9.4

Frame timing and time-slot synchronization

The IEC61334-5-1 protocol specifies a master-slave network with time-division medium

access: in order to properly communicate, all the nodes belonging to a network must share

the same “slot synchronization”.

The time division is fixed through the use of time-slots, corresponding to a physical frame

length of 45 bytes (i.e. 360 bits) with a total duration equal to:

●

15 mains cycles, at the 1200 bps operating speed (at 50 Hz);

●

7.5 mains cycles, at the 2400 bps operating speed (at 50 Hz).

The slot synchronization is first achieved by the master (i.e. ST7570 modem in 'Client'

mode) setting the time-slot starting at the mains zero-crossing instant. The frames

transmitted by the master will enable the slot synchronization of all other slave nodes (i.e.

ST7570 working in 'Server' mode): the reception of the sequence composed by PRE and

SSD will allow all the 'Server' nodes aligning their time-slots to the Client's time-slot.