Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 64)

Choosing Communication
Chapter 5
5-6
In Figure 5.4, we assume that station 1 wants to send the identical message
as shown in Figure 5.3 but to station 2. Station 1 has the token. Station 1
sends the message to station 2 and then passes the token on to station 2.
Now station 2 has the token but has not had time to generate a reply to
station 1. So station 2 sends any other messages it has queued and then
passes the token on to station 4. Stations 4, 5, and 1 all receive the token
in order and send any messages they have queued. The token then returns
to station 2, which then sends its reply to station 1. In this example, it took
an extra token pass around the network to complete the message
transaction even though the message was identical to the one shown in
Figure 5.3.
Internal Processing Time
Internal processing time depends on how busy a given processor on the
network is when sending or receiving a message.
For example, processor A has just received a READ request from
processor B on the network. If processor A already has three messages of
its own to send, the reply to the READ request from processor B will have
to wait until the station completes the processing of the messages queued
ahead of it.
Average DH+ Link Response Time Test Results
This section shows graphically the results of testing performed on a DH+
link where the number of stations and words sent in the message varies.
Figure 5.5 shows the average response time of messages of varying sizes
on a DH+ link with a varying numbers of stations. It also gives you an
idea of the typical response time you can expect on a given DH+ link.