FP Modem-EU Technical Manual
4.2 Multipoint Operation
The FP Modem-EU (multipoint mode) is equipped with a built-in terminating resistor that can be
activated/deactivated by a software command during the setup of the modem. For this purpose,
you connect the modem to a computer and use a standard terminal program (for example
Hyperterminal) to enter the special modem AT commands for setting up the multipoint
parameters. This is performed only one time. After that the parameters are permanently
memorized in the modem.
It is helpful to mark the modems which are setup with an active terminating resistor. Such marks
identify the modems which should be localized at the end of the straight cable later on.
Straight network
For a straight two wire cable you need a terminating resistor on each end in order to avoid
floating voltages, disturbance, noise, induction, high voltages and bad influences on the wire. If
a wire is not terminated by a resistor at the end of the cable, you might have problems with data
communication.
Star network
If you have a cable network in star topology, you cannot activate all terminating resistors at each
end of the star because it is not allowed to connect more than 3 or 4 active terminating resistors
to one communication cable. All terminating resistors are connected in a parallel manner to the
cable, which means that each additional terminating resistor reduces the data voltage level.
The FP Modem-EU also gives you the possibility to activate the terminating resistor only for the
time during which the modem is sending out data. In case of a star network with a master
modem in the middle, you can configure the master modem such that the terminating resistor is
always activated. All the other modems can be configured such that they only have an active
terminating resistor for when data are being sent. With this solution the momentarily used cable
ends are always terminated and you do not need more than two terminating resistors at a time.
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