Bradley Smoker PLC-5 user manual Block-Transfers, Ms/Word Overhead ms Rate kbps

C-8Maximizing System Performance

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:

block-transfer time = (number of words being transferred ms/word based on the communication rate) + overhead for the communication rate

Communication	ms/Word	Overhead (ms)
Rate (kbps)
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 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.

IMPORTANT

If you select the baud rate as 230.4 kbps, and you are using the serial port or a PLC-5 coprocessor, use channel 2 for better overall system performance.

Publication 1785-UM012D-EN-P - July 2005