Contemporary Control Systems ARCNET AI-USB/485 설치 매뉴얼 - 페이지 9

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485 DC-Coupled EIA-485

A removable 5-position screw connector (

for each DC-coupled EIA-485 segment. Two terminals are

provided for differential signal phase A and two for phase B.

The middle pin connects directly to chassis (the enclosure). A

DC-coupled segment can be up to 900 feet of IBM type 3 (or

better) STP or UTP cable, and as many as 17 nodes can occupy

a segment. Make sure the phase integrity of the wiring remains

intact. All phase A signals on the network interface modules

and hubs must be connected together. The same applies to

phase B. If shielded cable is used, the shield should tie to the

middle pin on the connector. Be sure the data rate is the same

for all network devices (see

Termination

Each end of the 485 segment must be terminated in the

characteristic impedance of the cable. An internal resistance

can be invoked with a jumper on the EIA-485 daughter board.

With the middle jumper inserted at E1 on the daughter board,

120 ohms of resistance is applied across the twisted-pair. With

the jumper removed, no termination is applied.

external termination instead, remove this jumper and install a

120-ohm 1/4 watt resistor across pins A and B of the screw

terminal connector. A value under 120 ohms is not recommended

because it may excessively load the EIA-485 transceivers.

Bias

In addition to termination, bias must be applied to the ends of a

twisted-pair network (and nowhere else) so that when the line is

floated, differential receivers will not assume an invalid logic

state. There are two precision bias resistors of equal value on

the daughter board, one tied to +5 V and the other to ground.

Each resistor has a jumper associated with it. If both jumpers

are installed, the resistor tied to +5 V is connected to the (+)

signal and the grounded resistor is connected to the (–) line.

The termination and bias rules are simple: If the AI-USB is at

the segment end, install all three jumpers at location E1 on the

daughter board (

To use external termination, remove the middle jumper at E1.

Figure 5

Figure 3

). Otherwise, remove all jumpers.

TD030500-0IB

Figure 1

and associated explanation).

) is provided

To apply