Bradley Smoker PLC-5 Communication Rate, Number of Rack Entries, Time Rate kbps, 57.6 115.2 230.4

Maximizing System Performance C-7

These three factors affect the remote I/O scan time:

•communication rate

•number of rack entries

•block-transfers

Communication Rate

The communication rate determines the time it takes for the scanner to communicate with each individual entry in its scan list. The following table lists the amount of time required to communicate to a device at each communication rate.

Communication	Time
Rate (kbps):
	(ms):	Note that these are full rack times.
		Smaller racks will decrease this time.
57.6	10
115.2	7
230.4	3

If four full-rack entries are in the scan list, the I/O scan for that channel at

57.6kbps is 4 x 10 = 40 ms. If you change the communication rate to 230.4 kbps, the I/O scan decreases to

4 x 3 = 12 ms.

Number of Rack Entries

You determine the total remote I/O scan time in the remote I/O system by this formula:

total remote I/O scan time = # of rack entries X time per rack-entries in the scan list (see on page 7)

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.

Publication 1785-UM012D-EN-P - July 2005