Rockwell Automation 1785-Lxxx Enhanced and Ethernet PLC-5 Programmable Controllers - Manual (Page 145)

Publication 1785-UM012D-EN-P - July 2005
Communicating with Devices on a DH+ Link 9-13
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,
as shown below.
Size and Number of Messages
A PLC-5 controller 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 as shown in the table below.
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 controller wants to send a 150-word message, it will have
to transmit two messages, possibly requiring multiple token rotations.
DH+ link
Max. 38 ms
with the token
Min. 1.5 ms
with the token
5
Station
Station
1
Station
2
Station
3
Station
4
Sending
Station Command Type Maximum Packet
Size (Data Words)
PLC-5 Typed READ/WRITE 114
PLC-5 Word range READ/WRITE 117
PLC-2 Unprotected READ/WRITE 121