BECKHOFF EtherCAT Technology Section I - Manual (Page 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
t Clk
Clock
Symbol
Manchester
Biphase L
IDLE
Data 0 0 N+ 1 0 1 0
Ethernet Preamble
1 N- 0 0 0/1
CRC
0/1
t
SOF
t
EOF
SOF
Figure 17: EBUS Protocol
6.3 Timing Characteristics
Table 21: EBUS timing characteristics
Paramet
er
Min Typ Max Comment
t
Clk
10 ns EBUS Clock (100 Mbit/s)
t
SOF
15 ns Positive level of SOF before falling edge
t
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.