Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 146)

Publication 1785-UM012D-EN-P - July 2005
9-14 Communicating with Devices on a DH+ Link
The number of messages a station has to send also affects throughput time.
For example, if a station has three messages queued and a fourth is enabled,
the fourth message may have to wait until the previous three are processed.
Message Destination
Throughput times vary depending on whether a receiving station can process
the message and generate a reply before it receives the token. The figure below
assumes that station 1 wants to send a message to
station 4.
Station
Station Station
Station
2
1
4
5
Message
2. Now station 1 must pass the token on to the next
next highest station number, which is station 2.
4. Station 4 can now reply to the
message from station 1. This
completes the message transaction.
Station
Station Station
Station
2
1
4
5
Message
1. Station 1 has the token. Only the
station that has the token can send a
message. Station 1 sends the message
to station 4.
3. 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.