Rockwell Automation 57C570 AutoMax PC3000 User Manual - Manual (Page 72)

4Ć14
4.4.1 Determining the Effect on the Remote I/O Scan
Time by the Data Communication Rate
The communication rate determines the time it takes for the PC3000
module to communicate with each enabled rack. Table 4.4 lists the
amount of time required to communicate to an enabled device at
each communication rate.
Table 4.4 Ć Communication Times at Different Communication Rates
Data Communication
Rate (kbps)
Time
(ms)
57.6 10
115.2 7
230.4 3
If four fullĆracks are enabled, the I/O scan at 57.6 kbps is
4 10 = 40 ms. If you change the communication rate to 230.4
kbps, the I/O scan decreases to 4 3 = 12 ms.
4.4.2 Determining the Effect on the Remote I/O Scan
Time by BlockĆTransfer Requests
A blockĆtransfer is an interruption of the normal remote I/O scan to
transfer a block of data to a specific I/O module. Most of the time
that the PC3000 spends in performing the blockĆtransfer is for the
handshaking that occurs between it and the blockĆtransfer module.
This handshaking is embedded in the discrete I/O transfer and has
no effect on the remote I/O scan. The remote I/O scan is affected
when the actual data transfer occurs.
The amount of time that the blockĆtransfer interrupts the remote I/O
scan depends on the number of words being transferred, the data
communication rate, and associated overhead.
Use this formula and the table below to calculate the blockĆtransfer
time:
block-transfer time = (number of words being
transferred ms/word based on the communication
rate) ) overhead for the communication rate
Communication
Rate (kbps) ms/Word Overhead (ms)
57.6 .28 3
115.2 .14 2.5
230.4 .07 2
For example, if the communication rate is 115.2 kbps and you want
to blockĆtransfer 10 words, the interruption of the remote I/O scan is:
(10 .14) + 2.5 = 1.4 + 2.5 = 3.9 ms
For the particular remote I/O scan in which the blockĆtransfer takes
place, 3.9 ms will be added to the remote I/O scan time.
NOTE: These are full rack
times. Smaller racks will someĆ
what decrease this time.