On-Demand Function; Special Devices Used In On-Demand Function - Mitsubishi Electric MELSEC FX Series User Manual

FX communication

8.10 On-demand Function

Data transmission between the computer and programmable controller is usually initiated by the
computer only. The on-demand function is used if there is a need to transmit data from the
programmable controller to the computer. The range of data registers containing the data to be
sent is specified in special data registers.
Notes
• This function can only be used when the configuration of the computer and
programmable controller is 1:1.
8.10.1

Special Devices Used in On-demand Function

The special data registers and auxiliary relays used in the on-demand function are explained in
the following table.
Device
On-demand handshake
M8127
signal
M8128 On-demand error flag
Flag for specifying word
M8129
or byte data format
On-demand head
D8127
device number register
On-demand data length
D8128
register
Notes
• The on-demand handshake signal is turned on when data transmission to the
computer is started from the programmable controller, and is turned off when
transmission of the specified data is complete.
This is used as an interlock so that multiple on-demand requests can not be issued
simultaneously.
Programmable controller
D8127
D8128
Program
D(n)
D(n+m-1)
Name
On during execution of on-demand
ON: on-demand data being transmitted
OFF: on-demand data transmission complete
On if there is error in specified value for data transmission in on-
demand
ON: error
OFF: no error
Word/byte data format for on-demand data.
ON: byte units (8-bits per data register)
OFF: word units (16-bits per data register)
The head device number of the data area in which the data to be
transmitted is stored. Set by programmable controller's program.
The data length to be transmitted by on-demand. Set by
programmable controller's program.
Head address
Data length
Computer
Description
Commands 8
8
8-15