Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 63)

Choosing Communication
Chapter 5
5-5
Message Destination
Throughput times vary depending on whether a receiving station can
process the message and generate a reply before that station receives the
token. Figure 5.3 assumes that station 1 wants to send a message to
station 4.
Figure 5.3
Message DestinationĊExample 1
Station
Station Station
Station
2
1
4
5
Message
Station 1 has the token. Only the station that has the token can send a
message. Station 1 sends the message to station 4. Now station 1 must
pass the token on to the next highest station number, which is station 2.
Station 2 has the token. Assume that station 2 has messages to send and
holds the token for 30 ms. During this time, station 4 has processed the
message from station 1 and has a reply queued up. When finished, station
2 passes the token on to the next highest station number, which is station 4.
Station 4 can now reply to the message from station 1. This completes the
message transaction.
In Figure 5.3, station 4 has had time to process the message and generate a
reply. But, that is not the case with station 2 in Figure 5.4.
Figure 5.4
Message DestinationĊExample 2
Station
Station Station
Station
2
1
4
5
Message