BSS 366T OmniDrive [94/112] Serial communications rs485

BSS 366T OmniDrive [94/112] Serial communications rs485
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FDS-366T
RS485 & Multidrop
Unlike RS232, that can only work with two devices and with cable lengths of
about 50 feet, or MIDI that has a similar cabling length restriction, RS485 can
be used to set up 'multidrop' networks of units that are capable of
communicating successfully over long distances - up to around 4000feet. If
you need to position Omnidrive units more than 50 feet apart (or away from
the controlling PC) then the RS485 serial ports should be used instead of either
MIDI or RS232.
RS485 allows up to a maximum of 32 devices to communicate at 'half-
duplex' on a single pair of wires (plus ground), known as '2 wire' mode.
Note: The RS485 standard also supports '4 wire full-duplex' links. These
installations are only applicable to extend an RS232 link with two RS485
converters, e.g. for use with the older FDS-355 Omnidrive or Minidrive
products, there is no need or facility to use 4 wire links with the FDS-366T.
There are two RS485 connectors built in to the FDS-366T and these are wired
in parallel to make connecting units easy. Cables to jump between units
should be made up using two male 9 way D-type connectors wired as follows:
pin 4 - to - pin 4
pin 5 - to - pin 5
pin 3 - Cable screen
pin 3 - to - pin 3 (Optional - only if ground drops encountered)
Although RS485 is a balanced system, it can only cope with a few volts of
difference between unit grounds. Pin 3 is intended to be connected for cable
screen but is also connected to the unit ground via an internal 100 ohm resistor
and can be used to join the adjacent two units' grounds together. Be aware that
this may cause ground loop problems in the audio and may be masking errors
in your system earthing.
To enable RS485 communication the Serial Port parameter in the Utilities must
be set to "RS485". Note that RS232 and RS485 cannot be used simultaneously.
Connecting two or more devices together using RS485 creates a 2 wire
'multidrop' network. In its most basic form this could be two FDS-366Ts
situated in different rooms or a FDS-366T connected to a PC using an RS232/
485 converter, further units can then be added to this network. However, the
network requires termination at its extremities and this is usually achieved by
wiring a 100-120 resistor across the data lines (pins 4 and 5). This is
probably best fitted inside the RS485 9 way D type connector on both the first
and last units in the chain. If a PC with RS232/485 converter is involved you
may find that there is either a solder link or dip switch setting available to
actuate termination (see the section on converters for more information).
For the network to operate correctly there needs to be a minimum of 200mV
between the two data lines (pins 4 and 5). This can be ensured by connecting
one set of 'biasing' resistors across pins 3 and 5 and, 4 and 6 using 620
resistors, usually near or at the beginning of the network. See the diagram on
the left for a recommended configuration.
Connections
Termination and Biasing
Serial Communications RS485

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