LTH Electronics MXD75 - Modbus RS485 Connection Details

Modbus RS485 Connection
Details
Modbus RS485 Interface
Operating Guide
- 9 -
Modbus Connections
Modbus RS485 Connection Details
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
135 to 165 at a measuring frequency of 3 to
20Mhz
Cable Capacitance <30pF/m
Core Cross-section >AWG22
Cable Type Twisted Pairs
Loop Resistance ≤100/km
Signal damping Max 9 dB over the entire cable cross-section
Shielding Copper braided shielding or braided shielding
and foil shielding
Note the following when designing the bus structure:
 Using cable type A and with a transmission rate of 9600 Baud, the maximum line length
(segment length) of the MODBUS RS485 system is 10000 meters. The total length of the
spurs may not exceed a maximum of 6.6 meters.
 A maximum of 32 devices are permitted per segment.
 Each segment is terminated at either end with a 120  terminating resistor (not
supplied).
 The bus length or the number of devices can be increased by introducing a repeater.
The MXD70 Series provides a Modbus interface via an Optional Output Card
MXD73 Modbus RS485 Output Card Connection Details