Can Interface; I/O Response Time - Omron CJ1W-CORT21 Operation Manual

• Set the number of input buffers with message command 2902 to the same amount as the number
of configured input buffers (that is the total number of buffers configured with message command
2905 or 2906).
• Configure the order of input buffers according to the frequency of received messages, i.e. in
descending order. Buffer 0 is configured with the identifier of the most frequent received message.
The least frequent message should be received in the buffer with the highest number.
• If the total number of input buffers is high and the unit will receive messages that are not config-
ured (especially if they are very frequent), configure these message using same logic from the
previous guideline.
5-1-4

CAN Interface

This section describes the performance of the physical layer of CAN and the performance of the
CAN interface of the User Defined CAN Unit. The following figure depicts the transmission of CAN
messages on the bus
 Message Delay Time
Every message has a certain message delay time which is mainly determined by the size of the data
field in the message. The CAN bit-stuff mechanism (after every 5 consecutive equal value bits, a bit
of the opposite polarity is added) can increase the message delay time by a maximum of 15%. The
following formulas give the minimum and maximum message delay time.
Message Delay Time (seconds) = number of bits in message / bit rate (bit/s)
Message Delay Time
Message Delay Time
Note The maximum number of data bytes = 8
5-1-5

I/O Response Time

The I/O response time (CAN Bus response time) is the time between an input being set (or reset) on an
Input Node and an output being set (or reset) on an Output Node, all under the condition that the Input
Node and Output Node are linked to the User Defined CAN Unit.
The figures below show the minimum and maximum I/O response time. In the bottom figure, the pro-
cessing of received messages is finished immediately after a CPU I/O refresh. The received application
data cannot be transferred to the CPU until the next I/O refresh. The other factors that can influence the
minimum and maximum I/O response times are explained below.
CJ-series User Defined CAN Unit Operation Manual for NJ-series CPU Unit (W517)
(seconds) = 44 + (8 x number of data bytes) / bit rate (bit/s)
min
(seconds) = trunc (47.8 + (9.6 x number of data bytes) / bit rate (bits/s)
max
5 Communications Timing
5
5-5