Yaskawa MP940 - Manual (Page 102)

4.2 Serial Communications Function
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4.2.5 Time Required for Transmission
This section explains the time required for signal transmissions between a master and slave,
taking the MEMOBUS protocol as an example.
„ Overview
In a MEMOBUS System, the time required for signal transmissions between a master and
slave can be roughly calculated from the following seven items.
• Processing time for a command message from the master
• Modem lag time at the master
• Command message transmission time
• Slave processing time
• Modem lag time at the slave
• Response message transmission time
• Time for master processing of response message
To calculate the total processing time required when multiple slaves are connected to the
same master port, calculate the time required for each individual slave and then total them.
„ External Time Requirement Standards
The times for the seven items are explained in detail below.
Processing Time for a Command Message from the Master
This is the time required for the computer to prepare a command message up to the MEMO-
BUS port. This time depends on the processing time for the particular master.
For a Machine Controller, it depends on the scan time and is normally one scan.
Modem Lag Time at the Master
This is the time from when the modem at the master receives a request-to-send (RTS) signal
from the master until a clear-to-send (CTS) signal is returned to the master.
With a Yaskawa modem, this time is 5 ms or less, so it can basically be ignored. If no
modem is used at all, then this time lag will not exist.
Command Message Transmission Time
This is the time required for the command message to be sent from the master’s communica-
tion port.
This time depends on the message length and the baud rate. It can be calculated by means of
the following equation:
Transmission time =
(Number of command message characters) x (Number of bits per character) x 1,000
Baud rate
(ms)