30751853-001 Honeywell

To achieve fast reliable data connections good quality screened twisted

pair cable should be used. The maximum achievable data rate is

limited by the cable capacitance and therefore the cable length should

be kept as short as possible. In dual highway configurations a separate

twisted pair should be used for each highway, however if required,

these may be contained within a single cable.

In order to ensure the correct operation and to meet European

Standards for RFI and EMC, it is recommended that all field cables

should be of the screened type with the cable screen connected at one

end only. If the screen is to be connected at the System 57 end, use

either the ground terminal of the DC Input Card, the cabinet using a

suitable metal cable gland or other suitable instrument earth point.

3.4.3 RS485/422 Transmission Line Termination

In RS485 or RS422 applications, the two wire transmission lines must be

properly terminated. The simplest form of termination is typically with a

120 ohm resistor connected across the differential input as follows:

a. In RS422 host systems, a terminator resistance is only required at

the receiver device located in the host and at the receiver device

located in the System 57 node at the far end of the cable.

b. In RS485 host systems a terminator resistance is required at the

transceiver device located in the host and at the transceiver device

located in the System 57 node at the far end of the cable. For dual

highway RS485 systems both highways should be terminated as

described.

Jumper links are provided on the System 57 RS485/422 Module to

make above termination easy (see Figure 1).

In general RS485 transmitter circuits are specified as being capable of

driving a minimum load resistance of 60 ohms, therefore no more than

two terminator resistors should be connected in parallel onto any one bus.

RS422 transmitter circuits are specified as being capable of driving a

minimum load resistance of 100 ohms, therefore no more than one

terminator resistor should be connected onto any bus.

3.4.4 RS485 Network Biasing Resistors

Note: In most systems network biasing resistors are not required. It is

recommended that these resistors are only added as part of a

troubleshooting procedure

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