14
model 3000 DeviceNet Module Manual (DN311/DN311A)
Chapter 1 Overview of the Devicenet Module
1
(2) Instruction execution time in accessing the DN311 from the S controller
In sending output data to slave devices from the S controller via the DN311, the
output data is written in the DN311 from the S controller by an MWRITE instruction. In
sending input data from a slave device via the DN311, the input data is read from the
DN311 by an MREAD instruction. Table 1-6 lists the instruction execution times of
the S controller.
The DN311 has an output data area of 378 words (area where output data to slave
devices is stored) and an input data area of 378 words (area where input data from
slave devices is stored). Table 1-7 lists the execution times for WRITE/READ
instructions by the S controller in accessing 378 words. A WRITE/READ instruction is
capable of batch transfer of 256 words maximum only. To transfer 378 words, for
example, the you may divide the data into two parts, say, 256 words and 122 words.
When input/output data with slave devices is smaller than 378 words, it takes less
time to execute an instruction. If the output data area or input data area is accessed
more than once, the time of executing instruction will be as much as the number of
accesses made to it. (The output data area and input data area are explained in 4.2.1
Input/output data areas.)
N: Number of words to transfer I/O access time: 3.80
µ
s
The values in Table 1-7 indicate the time required accessing the DN311. If two or more
DN311s are mounted, calculate the total time of executing instructions by adding the time
of access to each DN311.
If the total of the time lengths required for the S controller to access two or more DN311s
and for other than accessing the DN311s, that is, input/output and internal processing, is
greater than the response time of the system you require, mount the DN311s in separate
S controllers according to your system configuration.
Table 1-6 READ/WRITE Instruction Execution Time (
µ
s)
MREAD instruction
7.36 + (4 + N)
×
I/O access time
MWRITE instruction
7.24 + (4 + N)
×
I/O access time
Table 1-7 Maximum Execution Time for DN311 Access (ms)
(378 words for input/output)
MREAD instruction
1.48
MWRITE instruction
1.48
Total
2.96
