Cubemass DCI MODBUS RS485 Wiring
Endress+Hauser 19
4 Wiring
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Warning!
When connecting Ex-certified devices, see the notes and diagrams in the Ex-specific supplement to
these Operating Instructions. Please do not hesitate to contact your Endress+Hauser sales office if
you have any questions.
!
Note!
The measuring device does not have an internal disconnecting device. Therefore, assign a switch or
circuit breaker to the measuring device with which the voltage supply line can be disconnected from
the power system.
4.1 MODBUS RS485 cable specifications
In the EIA/TIA-485 standard, two versions (cable type A and B) are specified for the bus line and
can be used for all transmission rates. However, we recommend you use cable type A. The cable
specification for cable type A is provided in the following table:
Note the following points for the bus structure:
• All the measuring devices are connected in a bus structure (line).
• Using cable type A and with a transmission rate of 115200 Baud, the maximum line length
(segment length) of the MODBUS RS485 system is 1200 m (3936 ft).
The total length of the spurs may not exceed a maximum of 6.6 m (21.7 ft) here.
• A maximum of 32 users are permitted per segment.
• Each segment is terminated at either end with a terminating resistor.
• The bus length or the number of users can be increased by introducing a repeater.
Cable type A
Characteristic impedance 135 to 165 Ω at a measuring frequency of 3 to 20 MHz
Cable capacitance < 30 pF/m (< 9.2 pF/ft)
Core cross-section > 0.34 mm
2
(AWG 22)
Cable type Twisted pairs
Loop-resistance ≤ 110 Ω/km (≤ 0.034 Ω/ft)
Signal damping Max. 9 dB over the entire length of the cable cross-section
Shielding Copper braided shielding or braided shielding and foil shielding
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