Vaisala PTB220 - Manual (Page 46)

U SER ' S G UIDE _______________________________________________________________________
40 ___________________________________________________________________ M210194 EN -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
F IGURE 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 HI
LO
RX/TX
HI
LO
RX/TX
HI
LO
RX/TX
HI
LO
120R
33nF
DYNAMIC LINE TERMINATION REQUIRED
IF NO DEVICE AT THE END OF THE BUS
...........
F IGURE 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.