Danfoss VTT Series Applications And Installation Manual page 28

Figure 14
Balanced Data Transmission/
Differential Concept
Page 28 of 73
M-AI-VT-001-EN Rev.C.1
Overview of the VTT Compressor
Balanced Data Transmission
Differential Concept
The RS-485 system uses a master/slave
architecture where each slave device (the VTT
Series Compressors) has its unique address
and responds only to packets addressed to
this device. The packets are generated by the
master (system controller/connecting computer)
which periodically polls all connected slave
devices. Data travels over the single line in both
directions. Software prevents more than one
device from transmitting at the same time.
All of the devices must have drivers with tri-state
outputs (including the master). Tri-state (or
three-state) buffers allow multiple devices to
drive the same output; in this case, the output
is the twisted-pair cable they all share. They are
called tri- state because they have three output
states: high (1), low (0), and high impedance (Z).
The high-impedance state effectively isolates
the device output from the cable when it is not
transmitting.
A situation where more than one device assumes
the role of master is a violation of master/slave
architecture and could lead to unpredictable
behavior; for example, having the programmable
A basic RS-485 system requires an I/O driver
with differential outputs and an I/O receiver with
differential inputs. Since the signal is transferred
via a twisted pair of wires, if noise or interference
is introduced into the line, the voltage difference
(between the twisted-pair wires) of this
interference is almost zero. Because the input
to the receiver is differential, this interference
is eliminated. Differential inputs also ignore
different earth potentials of the transmitter and
the receiver.
Twisted-pair cables, coupled with correct
terminations (to eliminate reflections), allow data
transfer rates of 30Mbit/s with a cable length up
to 4,000 feet.
For correct operation of the transmitter and
the receiver, a return signal path between the
grounding of individual devices is required. The
ground shield is used for this purpose.
logic controller (PLC) connected via RS-485
connecting a laptop via RS-232 and trying to run
a bearing calibration.