Vaisala PTB220 User Manual download pdf (Page 46)

USER'S GUIDE_______________________________________________________________________

40 ___________________________________________________________________ M210194EN-A

The two-wire half duplex RS 485/422 serial interface is the

recommended way of connecting the PTB220 series barometers with

other intelligent transmitters (see

FIGURE 5-4). See page 93 for quick reference information on how to use

the RS485/422 interface of the PTB220 series barometers.

HOST

COMPUTER

PTB220

...........

RX/TX

120R

33nF

DYNAMIC LINE TERMINATION REQUIRED

IF NO DEVICE AT THE END OF THE BUS

...........

FIGURE 5-4 PTB220 series barometer connected to a two-wire half

duplex RS 485/422 interface

Grounding

The PTB220 series digital barometers use several ground potentials.

Internally, the critical measurement interface has its own stable

analogue ground. Digital circuitry uses the power supply ground of

the barometer. The housing of the barometer forms a static shield

ground for the barometer.

Power supply ground must be connected to the pin 7 of the barometer.

For the RS 232C serial interface, a separate ground is provided on pin

5 of the barometer, whereas for TTL level serial communication the

pin 7 is recommended. Use pin 7 as ground also for the pulse output in

the pulse output mode.

The housing can be grounded in two ways. As the shell of the D-

connector is in the same potential with the housing of the barometer,

the user can ground the housing by means of the connector shell of the

mating connector and the cable braid. In this case, the other end of the

cable braid must also be appropriately grounded. In addition, the EMC

stability of the barometer is further improved by grounding the

barometer housing directly on an electrically conductive mounting

surface with a serrated lock washer which breaks the painting of the

housing.

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Vaisala PTB220 User Manual Page 46