Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 135)

Chapter 10
Maximizing System Performance
10-3
Table 10.A
Communication Times at Different Communication Rates
Communication Rate (kbps) Time (ms)
57.6 10
115.2 7
230.4 3
If there are four full-rack entries in the scan list, the I/O scan for that
channel at 57.6 kbps is 4 x 10 = 40 ms. If you change the baud rate to
230.4 kbps, the I/O scan decreases to 4 x 3 = 12 ms.
Important: All devices on the network must support the baud rate you
chose and must be within the required cable lengths.
Number of Rack Entries
You determine the total remote I/O scan time in the remote I/O system by
taking the number of rack entries in the scan list and multiplying by the
time per rack at the baud rate that you are using (see Table 10.A). If one
channel has twice as many racks as another, for example, the scan time for
the first channel is twice as long.
To optimize this scan time, divide your I/O racks between multiple
channels. Place your most time-critical I/O on one channel, and non-
time-critical I/O on the other channel. Since all I/O channels are
independent, a long remote I/O scan on one channel will not affect the
remote I/O scan on another channel.
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 processor spends in performing the block transfer is for the
handshaking that occurs between the processor 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 transferring of data is occurring.
The amount of time that the block transfer interrupts the remote I/O scan
depends on the number of words being transferred and the baud rate:
Communication Rate (kbps) ms/Word Overhead (ms)
57.6 .28 3
115.2 .14 2.5
230.4 .07 2
Note that these are full rack times.
Smaller racks will decrease this time.