Vaisala PTB220 - Manual (Page 99)

A PPENDIX G ______________________O PTIONAL RS485/422 INTERFACE OF PTB 220 BAROMETERS
V AISALA _________________________________________________________________________ 93
APPENDIX G OPTIONAL RS485/422 INTERFACE OF
PTB220 BAROMETERS
The two-wire non-isolated half duplex RS 485/422 serial interface is
the recommended way of connecting the PTB220 series barometers
with other intelligent transmitters.
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
...........
At the ends of the serial bus there must be a PTB220 series barometer,
a dynamic line adapter (120 Ω resistor in series with a 33 nF
capacitor) or a line master. If a branch line is made with a junction
box, the branch should be shorter than 3 meters.
To use PTB220 series barometers on RS485/422 interface the user
must select half duplex operation, set an address for each barometer
and activate the POLL/CLOSE mode. The barometers can then be
polled by the SEND command or by using the OPEN/POLL
commands (see pages 12 and 48). The time interval setting allows the
user to enter a stop (S) command if a continuous outputting (R)
command has been given by mistake (see page 27).
NOTE 1. The sending of PTB220 is controlled with XON/OFF (software
handshaking).
2. The buffer of the barometer can be cleared with .
Example of required software commands:
>seri
9600
E 7 1 F
>seri H
(select half duplex serial operation)
9600 E 7 1 H
>intv 1 s
(select at least 1 second outputting interval)
Output intrv. : 1 s