Allen-Bradley PLC-5 User Manual page 294

C-8 Maximizing System Performance
block-transfer time = (number of words being transferred
on the communication rate) + overhead for the communication rate
Publication 1785-UM012D-EN-P - July 2005
Block-Transfers
A block-transfer is an interruption of the normal remote I/O scan in order to
transfer a block of data to a specific I/O module. Most of the time that the
controller spends in performing the block-transfer is for the handshaking that
occurs between the controller and the block-transfer 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 block-transfer interrupts the remote I/O scan
depends on the number of words being transferred, the communication rate,
and associated overhead:
Use this formula and the table below to calculate block-transfer time:
Communication
Rate (kbps) ms/Word
57.6 .28
115.2 .14
230.4 .07
For example, if the communication rate is 115.2 kbps and you want to
block-transfer 10 words, the interruption of the remote I/O scan is:
(10 x .14) + 2.5 = 1.4 + 2.5 = 3.9 ms
For the particular remote I/O scan in which the block-transfer takes place, 3.9
ms will be added to the remote I/O scan time.
If you select the baud rate as 230.4 kbps, and you are using
IMPORTANT
the serial port or a PLC-5 coprocessor, use channel 2 for
better overall system performance.
Overhead (ms)
3
2.5
2
ms/word based