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120Ω

Keep away from high noise sources as can as possible while wiring. In the distribution box, do not use the same

groove with the power wire. For the external environment, keep away from the equipment which produces

electromagnetic radiation.

Pay attention to the communication distance and select the appropriate communication interface. As the

specications of the RS-485 interface are much better than the RS-232, should use the RS-485 interface as far

as possible in industrial control systems. Even so, in the use of RS-485 interface, many things must be noted in

accordance with its specications.

Guidelines for the use of RS-485

A. The transfer wire need to use shielded twisted pair wire. Normal twisted pair wire can be used when

conducting short distance communication in low noise environment to cut down cost. But in high noise

environment, long distance communication or in occasions where high communication quality is required, the

dedicated transfer wire for RS-485 (like the Belden 9841) is recommended. It may make higher budget, but the

communication quality will be improved magnicently.

B. Make sure the principle of connect in sequence is followed when do hardware wiring. Do not use the T-type,

the radial distribution type or any other wiring method for the purpose of convenience.

7-1-2 Notes for Constructing Communication Systems

Connect in sequence wiring

T-type wiring Radial distribution wiring

C. Terminal resistances must be parallel connected to the two terminal devices of the whole communication circuit.

For the twisted pair wire used by RS-485 interface, the terminal resistances should choose 120 Ω 1/2 W resistors.

Terminal device Terminal device

All of the VS series communication cards have built-in terminal resistances, user can be enabled the function

by short connect the TR terminals. For those communication devices which have no built-in terminal resistances,

take special note during wiring to ensure that the external terminal resistance is well connected.

D. Although the RS-485 is a two-wires-style interface, when the distance between 2 communication devices is too

long, communication often fails for the earth electric potential difference of the 2 devices is too big. Thus we

normally recommend using the shield layer of the transfer wire to connect the SG terminals of the 2 devices, so

that the earth electric potential difference can be reduced, and to achieve the purpose of normal communication.

Shielded Twisted Pair Wire

E. When the number of serial connections at the RS-485 circuit has exceeded a certain amount (depends on the

specication of the devices connected, usually 32), an RS-485 amplier has to be added to the circuit.

F. According to the standard specications of the RS-485 interface, the longest communication distance is 1200

meters. When the RS-485 communication circuit exceeds this distance, the RS-485 amplier must be added to

increase the communication distance.

It is possible that one communication circuit to connect with different devices at the same time, thus when the

communication fails, carefully inspect whether all wirings are correct and stable, furthermore check the

conguration values of each device are correct. On the other hand, usually a complicated structure is more

difcult to nd out the problem. Therefore, can separate a device to do individual checking before connecting it

with many other devices, that for to make sure every device can work well.

Vigor VS Series User Manual

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