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.

Min. 1.5 ms with the token

Station

5

Station 4

Station 1

DH+ link

Station 2

Max. 38 ms

with the token Station

3

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.

Sending

Command Type

Maximum Packet

Station

 

Size (Data Words)

 

 

 

PLC-5

Typed READ/WRITE

114

 

 

 

PLC-5

Word range READ/WRITE

117

 

 

 

PLC-2

Unprotected READ/WRITE

121

 

 

 

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.

Publication 1785-UM012D-EN-P - July 2005

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Image 145
Bradley Smoker PLC-5 Nodes, Size and Number of Messages, Sending Command Type Maximum Packet Station Size Data Words