Beckhoff EtherCAT EL6001, Документация

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Product overview
EL600x, EL602x 16 Version: 4.6
Fig. 11: RS485 termination
The background is the different design of RS422/EIA-422 and RS485/EIA-485:
• RS422: 1 Rx → Tx n (maximum 10 receivers)
• RS485: n Rx → Tx m (maximum 32/128 devices, depending on the resulting bus loading)
Components for RS485 usually have a higher input impedance, resulting in lower bus load.
Termination with EL602x and BIAS resistors
The EL602x devices do not have integrated termination resistors, in order to enable operation in
bus mode. Any termination that may be required must be connected outside the terminal.
The EL602x devices feature integrated bias resistors < 1 kOhm, which bring the bus lines to defined
levels, even if the line is disconnected.
If several EL602x devices are connected in a bus, the parallel bias resistors may hamper the data
communication. In this case the EL6021-0021, which has significantly higher bias resistors, should
be used as an intermediate device.
Topology
The termination and the bias resistors generate a load on the bus. However, they are essential for
unambiguous bus levels and therefore have to be positioned with diligence. Ideally the RS422/485 bus
should be configured as a daisy chain or a simple chain, see
Fig. [
}
17] The following topologies may be
problematic:
• Star topologies: each end point should ideally be terminated, but this can lead to excessive bus loading
and ambiguous signal levels. Other potential issues are reflections and runtime variations.
• Intermeshed topologies: no clear end points, which means reflections and circulating currents are
possible.
Shielding/shield
NOTE
Do not use functional earth for discharge of residual currents or potential differences!
The EL60xx units offer a shielded connection for discharging EMC interference via the cable shield (FE,
functional earth). The shield must not be misused for discharging residual currents or potential differences.
The EL60xx units offer a shielded connection for discharging EMC interference via the cable shield (FE,
functional earth). The shield must not be misused for discharging residual currents or potential differences.