Beckhoff EtherCAT Technology Section I Manual de usuario descargar pdf (Pagina 56)

EBUS/LVDS Physical Layer

I-36 Slave Controller – Technology

6.2 EBUS Protocol

Ethernet/EtherCAT frames are Manchester encoded (Biphase L) and encapsulated in Start-of-Frame

(SOF) and End-of-Frame (EOF) identifiers. A beginning of a frame is detected if a Manchester

violation with positive level (N+) followed by a ‘1’ bit occurs. The falling edge in the middle of the ‘1’

indicates the SOF to the ESC. This ‘1’ bit is also the first bit of the Ethernet preamble. Then the whole

Ethernet frame is transmitted (including Ethernet SOF at the end of the preamble, up to the CRC). The

frame finishes with a Manchester violation with negative level (N-), followed by a ‘0’ bit. This ‘0’ bit is

also the first bit of the IDLE phase, which consists of ‘0’ bits.

EOF IDLE

Clk

Clock

Symbol

Manchester

Biphase L

IDLE

Data 0 0 N+ 1 0 1 0

Ethernet Preamble

1 N- 0 00/1

CRC

0/1

SOF

EOF

SOF

Figure 17: EBUS Protocol

6.3 Timing Characteristics

Table 21: EBUS timing characteristics

Paramet

Min

Typ

Max

Comment

Clk

10 ns

EBUS Clock (100 Mbit/s)

SOF

15 ns

Positive level of SOF before falling edge

EOF

15 ns

Negative level of EOF after last edge

NOTE: After power-on, a receiver which receives IDLE symbols cannot distinguish incoming ‘0’ bits from ‘1’ bits,

because it is not synchronized to the transmitters phase. Synchronization is established at the falling edge at the

end of the EBUS SOF, which indicates the center of the first preamble bit. After synchronization, idle errors can

be detected by the ESC.

NOTE: SOF is detected at a falling edge following a period of at least 15 ns (nominal) of positive level, EOF is

detected after a period of at least 15 ns (nominal) of negative level. I.e., the length of SOF and EOF can be even

longer.

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