Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 147)

Publication 1785-UM012D-EN-P - July 2005
Communicating with Devices on a DH+ Link 9-15
In the following figure, station 4 has had time to process the message and
generate a reply. However, in , station 2 does not have sufficient time to
process a MSG reply.
Internal Processing Time
Internal processing time depends on how busy a given controller on the
network is when sending or receiving a message.
For example, controller A has just received a READ request from controller B
on the network. If controller A already has three messages of its own to send,
the reply to the READ request from controller B will have to wait until the
station completes the processing of the messages queued ahead of it.
Station
Station Station
Station
2
1
4
5
Message
4. The token then returns to station 2, which
then sends its reply to station 1.
1. In this figure, we assume that station 1 wants to send the
identical message as shown in Figure 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.
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.
Station
Station Station
Station
2
1
4
5
Message
3. Stations 4, 5, and 1 all receive the
token in order and send any
messages they have queued.
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
10.2.
.