POSYC 4301/4302/4303 series industrial PCs
Installation, use and maintenance manual - EN
Rev. 1.02
28
Through appropriate repeater devices (maximum 9) it is possible to extend the connection and connect up to
126 devices.
Example of RS485 wiring
An additional conductor with respect to the couple indispensable to the communication that connects the mass-
es with each other significantly reduces the probability of failure of the non-isolated serial ports that could be
part of equipment with very different ground references.
7.4.3 Profibus DP port wiring
Like the RS485 port, up to 32 devices (including the master) can be connected to a single line via the Profibus
DP port, offering good noise immunity and a maximum transmission speed.
According to the specifications of the Profibus standard, the maximum length of the section is 1200mt at a max-
imum transmission rate of 93.75Kbps. It should be noted that the length of the section is inversely proportional
to the communication speed and that at the maximum transmission speed of the gate, or 12 Mbps, the length
of the section is 100 meters.
For correct operation it is necessary to use a suitable cable, terminating it appropriately. The
termination consists in connecting at the beginning and at the end of the section of the re-
sistors with a value of 120ohm; for the beginning and end of the section, the master and the
most remote slave are considered respectively (called 1 and 4 in the figure below). The
length of the cable connecting the intermediate line (called 2 and 3 in the figure below) must
be as short as possible (in any case below 1mt), otherwise the signal quality will be lost. The
RS485 serial ports of the POSYC do not have the termination resistors connected internally.
It is good practice to ground the shield of the cable used only at one end (preferentially from
the computer side) to avoid that potential differences between the two devices create a cur-
rent circulation on the wiring screen, thus introducing communication disturbances; in some
applications it is however possible to connect to both ends. The non-connection of the
screen to the ground instead (disconnected from both ends) nullifies the function of the
screen itself.
