Vaisala DMP248 Service Manual download pdf (Page 126)

APPENDIX 4 ________________________________________________________________________

120 ____________________________________________________________________ U263EN-1.4

simpler wiring. Data transmission is achieved by switching the loop

current on and off.

Normally, current flows through the loop(s) even when the DMP248

transmitter is not on, so switching one transmitter off does not affect

the other transmitters on the loop.

When the wires have been connected correctly, the voltage drop from

RX+ to RX- is below 2 V. If the wires RX+ and RX- or TX+ and TX-

are connected incorrectly, the voltage drop from RX+ to RX- or from

TX+ to TX- is below 1 V and the transmitter does not work. Even

then the current goes through the loop and the other transmitters can

be operated normally.

When the loop supply is current limited, the data lines can withstand

short circuit to ground and to each other. They do not survive

connection of supply voltage to the data lines.

The module must be mounted on the main board in the right direction.

It can be mounted in the wrong direction or to the wrong pins without

breaking the module; it simply does not work then. Reverse wiring of

RX+ and RX- or TX+ and TX- does not affect the module.

Network configuration

Single loop operation

Bi-directional data on one pair and galvanic isolation are the

advantages of the current loop. Single pair/dual pair use is configured

through wiring (see figure).

Supplying power from the same end to the loops prevents crossover

voltages.

• Connect wires to the transmitter’s serial connector.

• Check the wiring.

The following procedure must be repeated with all transmitters.

• Open the transmitter cover.

• Pull out the digital current loop module, if it is already

mounted.

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Vaisala DMP248 Service Manual Page 126