Modicon Modbus Plus Network Planning and Installation Guide

Modbus Plus Transaction Elements
Transaction Timing Elements
Token Holding
Time
Worst Case
Timing Examples
Each node holds the network token for a minimum length of time if it has no
transactions. The minimum token time is approximately 530 microseconds. The
token will be held for a longer time depending upon the quantity and size of pending
transactions.
Typical times are shown in the charts below. Each chart shows the types of
transactions the device can handle, how many concurrent transactions are
available, and the times required to process single and multiple paths. Times are
shown for small and large size transactions. All times are in milliseconds.
Note: The token holding times shown in the right column are worst-case times, with
all of the deviceís paths active, with all paths moving the full amount of data, and
with full queueing. With proper network design, these times should not occur in
practice.
The only types of transactions that you should consider for calculating the loading
on a properly-designed network are the data master paths and data slave paths,
with occasional queueing.
You should plan your network and application programming to avoid the worstcase
times. Use the formulas (see p. 75) to predict response times under various
loading conditions. See the guidelines (see p. 63) for designing your network to
avoid or minimize queuing in your application.
controllers
Transaction
Type
Two Registers 100 Registers
Available
Transactions
One
Transactio
n
All
Transactions
One
Transaction
MSTR DM
path
4 1.4 5.6 3.0 12.0
DS path 4 1.4 5.6 3.0 12.0
Dequeue
transaction to
slave path
4 1.4 5.6 3.0 12.0
PM path 1 1.4 1.4 3.0 3.0
PS path 1 1.4 1.4 3.0 3.0
Totals 8.4 21.0 18.0 45.0
All
Transaction
s
162 890 USE 100 00 November 2004