Wiring; Modbus Rs485 Cable Specifications - Endress+Hauser Proline Promass 84 MODBUS RS485 Operating Instructions Manual

Wiring

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22
4 Wiring
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 instrument does not have an internal disconnecting device. Therefore, assign a
switch or circuit breaker to the measuring instrument 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:
Cable type A
Characteristic impedance
Cable capacitance
Core cross-section
Cable type
Loop-resistance
Signal damping
Shielding
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 (4000 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.
135 to 165 Ω at a measuring frequency of 3 to 20 MHz
< 30 pF/m
2
> 0.34 mm , corresponds to AWG 22
Twisted pairs
≤ 110 Ω/km
Max. 9 dB over the entire length of the cable cross-section
Copper braided shielding or braided shielding and foil shielding
Proline Promass 84 MODBUS RS485
Endress+Hauser