Information Needed Before Programming; Programmable Controller Operation; Computer Notes - Mitsubishi Electric MELSEC FX Series User Manual

FX communication
7.2

Information Needed Before Programming

7.2.1

Programmable Controller Operation

The operation and the scan time of programmable controller using computer link is as follows.
While the programmable controller is running, access requests to the programmable controller
from the computer are processed on every END processing. The processing of Send or Receive
data is performed using interrupts.
Therefore, during the processing of Send or Receive, the scan time is extended; typically by
about 10%. The scan time can be monitored using the special data registers D8010 to D8012 of
the programmable controller.
Note:
- These are in units of 0.1 ms
7.2.2

Computer Notes

1 ) Conditions under which the transmission sequence is initialized.
The transmission sequence of the programmable controller is initialized during the following
conditions.
- When the power is turned on
- When data communication is completed normally
- When control code EOT or CL is received
- When a NAK control code is received
- After the time-out check time has elapsed (see section 7.4.4)
2 ) Occurrence of a framing error at the computer side
When a commercial RS-485 interface is used at the computer, if nothing is transmitted from
the programmable controller to the computer at the interface, a framing error may occur at
the computer.
Accordingly, read and skip the data at the computer until any one of STX, ACK, and NAK is
transmitted from the programmable controller.
3 ) NAK response from programmable controller
NAK response from the programmable controller to the computer is done when an error is
detected.
4 ) Command transmission from computer
When sending a command from the computer to the programmable controller using
dedicated protocol, only send the command after a gap of approximately two PLC scans
from the time the data communications required by the previous command was elapsed.
Caution:
When using RS-485 wiring (one pair, see section 2.3.3) remember to take account of/or
ignore the "echo" of the commands sent from the computer.
Computer link 7
7
7-3