053482-00 Honeywell Central control module

If the host device is not labelled or the polarity is not evident, some

experimentation may be necessary. The interface hardware can not

be damaged by reversing the polarity.

3.4.6 RS485 Connections

The System 57 interface supports up to 32 nodes connected to a

single RS485 highway. A secondary or backup highway is also available

if required. All A connections within a highway should be connected

together using one half of a twisted wire pair and all B connections

within the same highway should be connected together using the other

half of the twisted wire pair. Ideally the A and B connections should be

'looped through' each device. If a spur is necessary, the spur length

should be kept to a minimum at typically less than 1m (3ft). The data

ground return connection DGND of all devices must also be connected

together.

The voltage between the data grounds of the various devices must not

cause the common mode voltage rating of any device on the bus to be

exceeded. The data ground of each System 57 interface is isolated

from the System 57 ground to reduce earth loop current flow problems.

The cable screen should not be used as a data ground return and in

systems spread over a wide area the cable screen is best connected

to system ground at a single point only. A comprehensive wiring

example is shown in Figure 10.

3.4.7 RS422 Connections

The System 57 RS422 Interface supports up to 10 nodes connected to

a single RS422 highway. The host transmitter output connection A

should be connected to all device receiver inputs A’ using one half of a

twisted wire pair and the host transmitter output connection B should

be connected to all device receiver inputs B’ using the other half of the

twisted wire pair. The host receiver input connection A’ should be

connected to all device transmitter outputs A using one half of a

twisted wire pair and the host receiver input connection B’ should be

connected to all device transmitter outputs B using the other half of

the twisted wire pair. Ideally the A and B connections should be

‘looped through’ each device. If a spur is necessary, the spur length

should be kept at a minimum of typically less than 1m (3ft). The data

ground return connection DGND of all devices must also be connected

together.

The voltage between the data grounds of the various devices must not

cause the common mode voltage rating of any device on the bus to be

exceeded. The data ground of each System 57 interface is isolated

from the System 57 ground to reduce earth loop current flow problems.

The cable screen should not be used as a data ground return and in

systems spread over a wide area the cable screen is best connected

to system ground at a single point only. A comprehensive wiring

example is shown in Figure 11.