Bosch Appliances LBC 325X Network configuration, Cable length, Cable termination, Cable type

Intellivox DDC Installation Manual Appendix EN 18

Network configuration.

A network is composed of one master device and one or more slave devices. A master device can exist of a PC running the RS-485 network WinControl software and a RS-232 to RS-485 converter. All slave devices are wired in parallel, the master device is connected with the AB and YZ terminals interchanged (see Fig 14).

Because all slave devices share the same bus, the outputs (YZ) of these units are only enabled during transmission of data from the unit to the master. The implemented network protocol sets the output of all other units in a high impedance state during this period to prevent conflicts. For this reason each device has its own network address.

The official RS-485 standard limits the amount of nodes to 32. This is mainly because of the low resulting network impedance when all units are connected. The chassis is equipped with a new generation high impedance RS-485 transceivers, increasing the maximum node count from 32 to 256.

The RS-485 network protocol limits the amount of slave addresses to 126 and allows one master device.

Cable length

The maximum cable length is heavily depending on the cable characteristics and the baud rate. The RS-485 network baud-rate is 19200 bits/s which is relatively low for the RS-485 transmission path. Using low capacity low resistance cable the maximum length specified for safe transmission is 1500 m. If the distance is significantly longer, a network repeater will be needed.

Cable termination

According to the official RS-485 standard, the network should be terminated with 120 OHM resistors at both ends of each pair, while the length of the stub lines should not exceed 7 m. The RS-485 transceivers, which are implemented in the chassis, are slew rate limited which minimizes reflections from open cable lengths. This fact, in combination with the relative low baud-rate, makes the network very tolerant to stub lengths or improper termination.

In practice, termination resistors will only be needed in combination with large cable lengths and large stub lengths.

Cable type

The required cable type for implementation of the network is twin twisted cable of which both twisted pairs are individually shielded.

An example of a preferred extended range cable type is:

Type: BELDEN ‘Datalene’ series nr. 9729 2-pairs

Characteristic impedance :	100 Ohms
Core to core capacity :	41 pF/m
Core to shield capacity :	72.5 pF/m
DC resistance core :	78.7 Ohms/km.
DC resistance shield :	59.1 Ohms/km.

This cable has a foil shielding with drain wire and a tinned litze core.

Fig 15 Connection of twin twisted network cable to the unit

Alternative cable types are

:Belden 1902A or equivalent.

:BICC Brand-Rex H9002 AWM 2493

Bosch Security Systems July 2003