Communication Cable Laying Recommendations; Instruments With An Rs-485 Interface (2 Wire) - Athena C10 Communications Manual

6.2 Communication Cable Laying Recommendations

In order to minimise interference caused by the external environment to
serial communication, and thus obtain maximum efficiency between the
supervisor and the instruments, a few essential technical precautions
must be taken.
The most important and easiest to implement of all is to separate the
power or power supply lines from the communication lines and lay them
as far as possible from remote-controlled switches, electromagnets,
powerful motors, etc. The same rule applies to the control panel in that it
is pointless to cable the control panel perfectly and then haphazardly
"throw" the cables into the channel or vice versa. If the communication
cables are extended to another control panel or other equipment, leave a
space in the terminal board, isolated from all the other cables (normally
towards the sides).
The type of cable used is of fundamental importance for the functioning of
the entire system. The most important condition to be respected is the
cable's capacity per meter (pF/m). The lower the capacity of the cable is
the longer the line can be. Consequently, power cables, shielded coaxial
cables and general channel cables are to be avoided under all
circumstances in that they have an extremely high capacity per metre. In
addition, to ensure high interference rejection, the cables must be twisted
and preferably provided with a metal shield to be connected to an
efficient ground socket (on one side only).
Two examples of cables with suitable characteristics produced by Belden
are indicated below:
A) Belden code 9729
B) Belden code 9502

6.3 Instruments with an RS-485 interface (2 wire)

The line requires twisted cable with a characteristic impedance of about
120 Ω.
Normally this type of connection uses a standard 4-wire communication
interface. The transmission and reception signals for the computer and
the communicating device (Tx+Rx+ and Tx-Rx-) can be paralleled,
resulting in a single duplex connection RTx+ and RTx-.
The communication port cannot usually work if it is connected in this way
as every time the supervisor is transmitting a message, it is also present
on the receiving port before awaiting the reply from the other devices. To
prevent this problem occurring, the supervisor uses the RTS (Request To
Send) signal from the communication port. Before beginning a
transmission, the supervisor "raises" the status of its RTS signal to inhibit
its receiver. When the transmission has ended, the supervisor brings the
RTS signal back to "zero" to re-enable its receiver. In the same way, the
devices connected to the supervisor must be able to manage the
direction of the message that is flowing, otherwise the communication will
fail. ATHENA CONTROLS controllers include this ability in their software.
There are commercially available interfaces for handling the RTS signal
at the hardware level, so that it is completely transparent and not
required by the communications software.
It is strongly recommended that the two ends of the link between the
various devices is correctly terminated, as shown in the following drawing
to demonstrate the principle.
Z = 100 Ω
pF/m = 41
Z = 150 Ω
pF/m = 98
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