Rockwell Automation 1785-LTx,D17856.2.1 Classic PLC-5 Programmable Controllers Users Manual - Manual (Page 62)

Choosing Communication
Chapter 5
5-4
Nodes
Nodes affect transmission time in the following ways:
During one complete token rotation, each node on the DH+ link
receives the token whether or not it has something to send.
Each node spends from 1.5 ms (if it has no messages to send) to 38 ms
(maximum time allotted) with the token, assuming there are no retries
(Figure 5.2).
Figure 5.2
Token Passing
Station
Station Station
Station
Station
DH+ link
Max. 38 ms
with the token
Min. 1.5 ms with
the token
1
2
3 4
5
Size and Number of Messages
A PLC-5 processor encodes messages into packets for transmission on the
DH+ link. The maximum number of data words in a packet depends on
the sending station and command type. This limit comes from the network
protocol, which limits a station to transmitting a maximum of 271 bytes
per token pass. A station can send more than one message in a token pass,
provided that the total number of combined command and data bytes does
not exceed 271.
If a message exceeds the maximum packet size allotted, however, the
sending station will require more than one token pass to complete the
message. For example, if a PLC-5 processor wants to send a 150-word
message, it will have to transmit two messages, possibly requiring many
token rotations.
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.