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ABB | E-kit
RS-485 serial communication interface (COM1 port)
Type of cable required
Shielded twisted pair cable is required. ABB specifies Belden 3105A cables, but other types with equivalent
characteristics can be used.
Twisted pair cable is formed by two insulated conductors twisted together. This improves the level of immunity
to electromagnetic disturbances. The cable forms a series of consecutive coils, each pointing in the opposite
direction to the next. If there were a magnetic field in the vicinity, it would pass through each pair of coils in
opposite directions and its effect would consequently be very much reduced. Theoretically, the effect on each
coil is exactly opposite to that on the next, thus the resulting effect is annulled.
The shielding can be “braided” (formed by a woven lattice of conductor wires) or the “foil” type (formed by a
sheet of metal wound around the conductors): the two types are equivalent.
Termination resistor
To avoid signal reflection, a 120 Ohm termination resistor must be fitted to each end of the main cable.
Installation of termination resistors can only be omitted if the main cable is less than 50 m in length.
Note: there is no internal termination resistance in ABB SACE new Emax, X1 air and Tmax molded-case
devices and in the majority of ABB devices.
Pull-up and pull-down resistors
Two resistors, equal to 470 Ω, are integrated into connector COM1 of E-kit. They maintain a high level
defined in the Modbus network, even when data exchange is not taking place. To activate them, just connect
terminals 1 to 2 and 3 to 4 as shown in the diagram below.
1. Positive terminator
2. Modbus M+
3. Modbus M-
4. Negative terminator
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