Omron CJ1W-CORT21 Operation Manual page 41

 Serial Bus
A number of different data rates are defined from 1Mbps to 10kbps. Cable length depends on the
data rate used. The maximum line length is 5 km and the minimum is 25 meters at 1Mbps. Termina-
tion resistors are used at each end of the cable. The worst-case transmission time of an 8-byte
frame with an 11-bit identifier is 134 bit times (134 microseconds at the maximum baud rate of
1Mbps).
 Transmission Speed
Transmission speeds between 10 kbps and 1000 kbps can be selected as shown in the table below.
One unique transmission speed must be selected for all devices on the bus when the system is
commissioned.
Baud rate (kbps)
10
20
50
125
250
500
800
1000
Note For bus lengths greater than 1 km, a bridge or repeater device is recommended.
 Cable Length
The maximum cable length values depend on the transmission speed and are based on a
DeviceNet cable. The length can be increased by the use of repeaters. However, it is not recom-
mended to use more than three repeaters in series in a CAN network.
 Bit Time
The User Defined CAN Unit supports bit rates in the range of 10 kbps up to 1000 kbps. Every unit in
a CAN network has its own clock generator, usually a quartz oscillator. The frequency of this oscilla-
tor determines the bit rate of the unit and therefore the reciprocal of the bit rate, also called the bit
time. The timing parameter of the bit time (i.e. the sample point) can be configured individually for
each CAN unit, creating a common bit rate even though the CAN units' oscillator periods (fosc) may
be different. The nominal bit rate is the number of bits per second transmitted in the absence of
resynchronisation by an ideal transmitter. The nominal bit time is the reciprocal of the nominal
bitrate:
• nominal bit time = 1 / nominal bit rate
The frequencies of these oscillators are not absolutely stable and small variations are caused by
changes in temperature or voltage and by deteriorating components. As long as the variations
remain inside a specific oscillator tolerance range, the CAN nodes are able to compensate for the
different bit rates by resynchronizing to the bit stream. According to the CAN specification, the nom-
inal bit time is divided into four segments. The Synchronization Segment, the Propagation Time
Segment, the Phase Buffer Segment 1, and the Phase Buffer Segment 2. Each segment consists of
a specific, programmable number of time quanta. The length of the time quantum (t
basic time unit of the bit time, is defined by the CAN controller's system clock f
Prescaler (BRP): t
nization Segment Sync_Seg is that part of the bit time where edges of the CAN bus level are
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
Distance/segment (meters)
5000
2500
1000
500
250
100
50
25
= BRP / f . Typical system clocks are: f
q sys
1 Features and System Configuration
sys
= f or f = f
sys osc sys
1
), which is the
q
and the Baud Rate
/2. The Synchro-
osc
1-7