Micromod MOD: MODBUS RTU Communications Guide - Manual (Page 9)

MODBUS RTU
COMUNICATIONS GUIDE
5
SOFTWARE BASICS
MASTER/SLAVE RELATIONSHIP
A MODBUS RTU system consists of a Master and one or more Slave devices. Multiple Masters are not
permitted on the same network. The Master is responsible for initiating all communications, therefore, no
peer-to-peer capability is supported. With some hardware, it is possible to dynamically switch the device
between Master and Slave modes. This capability allows multiple Masters, though not simultaneously. With
special hardware, such as Phoenix Digital’s MPE Plus, having multiple Masters is possible, though it limits the
communications to the basic “Standard” MODBUS commands. Using the MPE Plus will block extended
MODBUS communications, including the Application Builder.
MESSAGE RESPONSE TIMES
The MODBUS RTU protocol relies on precise timing for reliable communications. The message structure is
such that a 3.5 character or greater pause will be interpreted as the end of a message. The actual time
varies with the baud rate. Conversely, most devices require a defined minimum amount of time between
messages. Another factor to consider is that each device has its own response time. This response time can
be anywhere from a few milliseconds to a few hundred milliseconds. The Host must be configured to allow
adequate time for the slowest device to respond.
INSTRUMENT RESPONSE TIMES TO HOST COMMANDS
These times represent the maximum time from when the instrument receives a request from the master, to
when it begins to send the response.
Table 2. Instrument Response Times
Device Max Response Time Device Max Response Time
C100 250 mSec C1900 250 mSec
C150 250 mSec Modcell MLP 60 mSec *
C200 250 mSec 2050R 100 mSec
C300 180 mSec MOD30ML 60 mSec *
C310 160 mSec MR250 125 mSec
C500 125 mSec PR100 90 mSec
* NOTE: The response times for Modcell MLP and MOD30ML are for read
messages. Write messages may take longer. These instruments have a
configurable Write Message Timeout parameter.