Lenze E94P PositionServo Modbus - Manual (Page 11)

11 P94MOD01C
Installation
3.3 Electrical Installation
3.3.1 Cable Types
Due to the high data rates used on Modbus networks it is paramount that correctly specified quality cable is
used. The use of low quality cable will result in excess signal attenuation and data loss.
For EIA-485 it is recommended to use a good quality shielded twisted pair cable with characteristic impedance
of 120
W.
For Ethernet it is recommended that a minimum specification of CAT5e UTP cable (unscreened) is used.
However, for environments that high levels of electrical noise STP (screened) cable is recommended.
3.3.2 Network Limitations: EIA-485
There are several limiting factors that must be taken into consideration when designing a Modbus RTU network,
however, here is a simple checklist:
• Modbus RTU networks are limited to a maximum of 247 nodes.
• Only 32 nodes (based on each node having a load impedance of 1 unit) may be connected on a single
network segment. Certain Modbus EIA-485 masters may only be able to support a fewer number of nodes
(i.e., 8, 16). Refer to the documentation for the Modbus master in use.
• A network may be built up from one or more segments with the use of network repeaters.
• Maximum network segment length is 1200 meters for baudrates up to and including 19200bps. Certain
Modbus masters may be limited to shorter runs. Refer to the documentation for the Modbus master in use.
• Minimum of 1 meter of cable between nodes.
• Use fiber optic segments to:
• Extend networks beyond normal cable limitations.
• Overcome different ground potential problems.
• Overcome very high electromagnetic interference.
• EIA-485 is a linear daisy chain topology. Both ends of the network segment must be terminated by a 120 W
±1% resistor.
3.3.3 Network Limitations: Ethernet
There are several limiting factors that must be taken into consideration when designing a Modbus TCP/IP
network, however, here is a simple checklist:
• Modbus TCP/IP networks are limited to a maximum of 255 nodes per subnet (based on a Class C addressing
system).
• Hubs are not recommended for general use as they contribute in creating network data collisions (ports on
a hub do not route data direct to other ports but instead all ports are open to receive data from every port)
and as such will cause additional delays in transmissions while the re-attempts are carried out.
• Switches are the recommended solution for connecting a multi-node network as they route network data
direct from port to port (collisions may occur during network start-up or when a device is connected and
the correct port routing is established) and therefore reduce the possibility of collisions.
• “Office grade” Ethernet equipment does not generally offer the same level of noise immunity or robustness
as “industrial grade” Ethernet equipment.