User`s manual

FX communication
Commands 8
8-15
8
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.
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 Name Description
M8127
On-demand handshake
signal
On during execution of on-demand
ON: on-demand data being transmitted
OFF: on-demand data transmission complete
M8128 On-demand error flag
On if there is error in specified value for data transmission in on-
demand
ON: error
OFF: no error
M8129
Flag for specifying word
or byte data format
Word/byte data format for on-demand data.
ON: byte units (8-bits per data register)
OFF: word units (16-bits per data register)
D8127
On-demand head
device number register
The head device number of the data area in which the data to be
transmitted is stored. Set by programmable controller’s program.
D8128
On-demand data length
register
The data length to be transmitted by on-demand. Set by
programmable controller’s program.
Program
Programmable controller
Computer
D8127
D8128
D(n)
D(n+m-1)
Head address
Data length
Notes
• This function can only be used when the configuration of the computer and
programmable controller is 1:1.
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.