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M-AI-VT-001-EN Rev.C.1
Table 8
RS-485 Specification
Specification
RS-485
Mode of Operation
Differential
Total Number of Drivers and Receivers on One Line
1 Driver
32 Receivers
Maximum Cable Length
100m (328 feet)
Maximum Data Rate
30Mb/s
Maximum Driver Output Voltage
-7V to +12V
Driver Output Signal Level (Loaded Min)
Loaded
±1.5V
Driver Output Signal Level (Unloaded Max)
Unloaded
±6V
Driver Load Impedance
54Ω
Max Driver Current in High Z State
Power On
±100 milli Amp
Max Driver Current in High Z State
Power Off
±100uA
Slew Rate (Max)
Receiver Input Voltage Range
Receiver Input Sensitivity
Receiver Input Resistance
n/a
-7V to +12V
±200mV
> or =12kΩ
Overview of the VTT Compressor
RS-485 Cable
Specifications
CIM Circuit Grounding
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).
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.
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.
The most likely cause of voltage is insufficient
insulation of the external circuit.
