Vaisala HMP228 - Manual (Page 85)

HMP228
M210282en-A Appendix 4: RS 485/422 serial port module
79
2. OPERATION
The HMP228 transmitters can either be given an address or operated without
an address. Both single and dual loop wiring with half duplex connection can
be used. No address is needed when only one HMP228 transmitter is used;
when several transmitters are connected to the same line, each transmitter
must be given an address in the initial configuration.
A single transmitter can get its operating voltage from the master or it can
have its own (floating) power supply or it has the power supply module in use.
The serial line structure is a parallel interfaced chain (daisy chain). At the ends
of the serial line there must be a HMP228 transmitter, dynamic line adapter
(120 ohm resistor in series with a 33 nF capacitor) or line master. If a branch
line is made with a junction box, the branch should be shorter than 3 meters.
When connecting the device, follow the instructions given in the figure in
Chapter 1.
TX HI
TX LO
TWISTED PAIR WIRING
-
HOST COMPUTER
DUAL LOOP WIRING
TWISTED PAIR WIRING
HOST COMPUTER
RX LO
RX HI
TX HI
TX LO
RX LO
RX HI
TX HI
TX LO
RX LO
RX HI
TX HI
TX LO
RX LO
RX HI
TX HI
TX LO RX LO
RX HI
120R
33 nF
TX HI
TX LO
RX LO
RX HI
120R
33 nF
120R
33 nF
NOTE: DYNAMIC LINE TERMINATION
REQUIRED IF NO DEVICE
AT THE END OF THE BUS.
NOTE: DYNAMIC LINE TERMINATION
REQUIRED IF NO DEVICE
AT THE END OF THE BUS.
HMP228
transmitters
1 to n pieces
HMP228
transmitters
1 to n pieces
SINGLE LOOP WIRING