Videotec DCT - Manual (Page 16)

3.2 RS485 and types of systems
The RS485 communication channel consists of a 2-wire line with a maximum length from end to end of
1200m.
Termination of the RS485 lines prevents signal reflection along the cable, and should be inserted in each of
the devices situated at the ends of the line
Systems can be of different types, therefore the way of terminating the lines will also be different.
In the following diagrams, the devices requiring termination are indicated by the
# # # # # # # # # symbol.
Configuration Description Example
Star /
Single lines Each connection between two devices has a
separate communication line, with a max.
length of 1200m.
All devices should be terminated, because
each appliance is connected to the end of a
line.
Backbone A single line is used, and the transmitters
can be placed in any position along it. The
two ends of the line (keyboard K and
receiver R3 in the example) are terminated;
the other devices (R1 and R2) are not
terminated. The maximum length of the line
is 1200m.
The specifications for the RS485 standard
allow at least 32 devices to be connected
along the same line.
Line with stub A certain number of stubs can be shunted in
parallel to the standard RS485 line, for
connection to other devices. The stubs are
not at the ends of the line, should not be
terminated and they should be very short in
length (of the order of a couple of metres).
The specifications for the RS485 standard
allow at least 32 devices to be connected
along the same line.
Devices in a
chain The devices are connected in pairs using
single lines. These should be terminated at
the ends. This type of configuration can
only be made when the devices used have
separate input (reception) and output
(transmission) channels, such as The
VIDEOTEC DTRX1 and DTRX3 receivers.
The received signal is “cleaned” and then
sent to the next device. If one device is
blocked, communication is cut off to the
devices that follow.
The maximum total length is equal to the
number of lines multiplied by 1200m for
each stage.
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