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Cable Networks/Permanent Connections
4.2.3 Error Correction in Multipoint Mode
Multipoint communication is only possible in V.23 line modulation mode. In multipoint mode no
automatic error correction is done (in contrast to dial-up or leased line mode).
In multipoint mode the modems do not execute an error handling protocol, i.e. the units that are
connected to the RS232C interface (PC or PLC serving the modem) have to detect and correct
possible transmission errors.
If for example the MEWTOCOL protocol is used in a multipoint modem network, the following
items must be taken into account:
•
If MEWTOCOL data are transferred via modem, the remote station can determine
whether the data packet has been transmitted correctly over the line or not by doing a
checksum.
•
If an error is detected, the remote station reports it to the sender with the respective
MEWTOCOL command. This gives the sender the chance to repeat the transmission of
the data packet. In this way error correction is realized by the MEWTOCOL, one level
higher than the modem.
•
But if MEWTOCOL multi-frame communication (multiple data packets for PLC program
download) is used, it is not possible to resend a single data packet. This means that the
program download is aborted if there is ONE single transmission error! The following
items helps to reduce the possibility of data transmission errors (FPWIN Pro program
download):
Prevent line disturbances (communication line not parallel to power lines)
-
Optimize the usage of the terminating resistor (see "Terminating Resistor" on page
-
64)
Set up FPWIN Pro to use short program download packets:
-
Locate the file "NAIS_MewPLC.dat" in the folder "\\Program Files\NAIS MEWNET\"
Open file with Notepad for editing
Find the section for your PLC (e.g. [FPSIGMA])
Find the parameter "EXTCMD=...."
Set parameter to ZERO: "EXTCMD=0"
Save "NAIS_MewPLC.dat" and end Notepad
Do not forget to set the communication answer timeout in FPWIN Pro high enough,
-
e.g. perhaps 25s. See FPWIN Pro menu 'Communication Settings'.
4.2.4 Terminating Resistor
Activating a terminating resistor within the modem will optimize the electrical characteristics of a
multipoint communication cable network.
Command
Function
AT&N0
Terminating resistor only when sending
AT&N1
Terminating resistor always off
AT&N2
Terminating resistor always on
64
FP Modem-EU Technical Manual
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