Beijer-electronics EPC TA100 AM EN Manual de usuario descargar pdf (Pagina 21)

Additional Installation Tips

6.7 CableandBusTerminationRS485

• Use shielded and twisted pair cable. The pair capacitance may not exceed 52.5

pF/m and area at least 0.25 mm

(AWG 24), if you want to use the maximum

transfer distance and maximum transfer speed.

• 0 V, the reference voltage for communication should be includ ed in

the cabling. With two-way communication use two pairs; one pair for

communication and one pair for 0 V.

• The shield must be grounded at one end. The other end is usually grounded,

but with longer distances or when there is a difference in the ground potential,

theshieldshouldbeconnectedtothegroundvia0.1uF/250Vplastic

capacitor to prevent ground current in the braided shield. A number of

manufacturers recommend that the shield be grounded at each node. Various

manufacturers have different systems for bus termination. The RS485

standard does not describe how the “Fail Safe” f unction would be carried out,

justthatthesystemshouldbeabletohandletheerror.

Depending on the recipients’ design, the bus wires may be on the same level or

require pull-up or pull-down to ensure that no faulty signals are detected when the

bus is in resting mode (all transmitters are disconnected).

1 23

+5 V

0 V

1 K

120 ohm 120 ohm

120 ohm

+5 V

0 V

1 K

VCC

(120 ohm)

Inside Panel PC

Panel PC

RS422

CAB8CAB8 Bus

RS485

Shield

Bus termination

5358

1 2 3 4 5 6 7 8

Some (older) operator panels had pull-up and pull-down resistance except for

the actual bus termination at 120 ohm, similar to Westermo and Proﬁbus. See 1

in drawing above.

The Panel PCs have another type of recipient, so-called built-in “Fail Safe”, where

simple bus termination resistance is sufﬁcient. See 2 in drawing above.

If other nodes on the RS485 network require pull-up and pull-down and the Panel

PC is at one end of the loop, one of the following procedures can be carried out:

• Connect two 1k/0.25 W resistors in the 25-pole D-sub contact. See 3 in

drawing above. Set jumper pins 6-19.

• Use CAB8. It offers the option of bus termination with pull-up/-down. It

isalsoeasytoconnectthebuscableviathescrewterminalblock. See4in

drawing above.

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Beijer-electronics EPC TA100 AM EN Manual de usuario Pagina 21

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