Rs-485 Cable Specifications; Cim Circuit Grounding - Danfoss VTT Series Applications And Installation Manual

Table 8
RS-485 Specification
RS-485 Cable
Specifications

CIM Circuit Grounding

Overview of the VTT Compressor
Specification
Mode of Operation
Total Number of Drivers and Receivers on One Line
Maximum Cable Length
Maximum Data Rate
Maximum Driver Output Voltage
Driver Output Signal Level (Loaded Min)
Driver Output Signal Level (Unloaded Max)
Driver Load Impedance
Max Driver Current in High Z State
Max Driver Current in High Z State
Slew Rate (Max)
Receiver Input Voltage Range
Receiver Input Sensitivity
Receiver Input Resistance
When choosing a transmission line for RS-485, it
is necessary to examine the required distance of
the cable and the data rate of the system. Losses
in a transmission line are a combination of AC
loss (skin effect), DC conductor loss, leakage, and
AC loss in the dielectric. In high-quality cable, the
conductor losses and the dielectric losses are on
the same order of magnitude.
The maximum Modbus cable length between the
compressor and the system controller should not
exceed 100 meters (328 feet).
Improper grounding or voltage in circuits
connected to the CIM board can lead to
component failures. In particular, the interlock
and analogue output circuits are sensitive to
improperly connected circuits.
32 Receivers
100m (328 feet)
-7V to +12V
Loaded
Unloaded
Power On ±100 milli Amp
Power Off
-7V to +12V
While the RS-485 specification does not specify
cabling, the recommendation is 24 AWG shielded
twisted-pair cable with a shunt capacitance of
16 pF/ft. and 100Ω impedance. Another choice is
the same cable commonly used in the twisted-
pair Ethernet cabling. This cable is commonly
referred to as Category 5 cable. The cable has a
maximum capacitance of 17 pF/ft. (14.5 pF/ft.
typical) and characteristic impedance of 100Ω.
The most likely cause of voltage is insufficient
insulation of the external circuit.
M-AI-VT-001-EN Rev.C.1
RS-485
Differential
1 Driver
30Mb/s
±1.5V
±6V
54Ω
±100uA
n/a
±200mV
> or =12kΩ
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