Vaisala HMP240 - Manual (Page 105)

HMP240
M210300en Appendix 3: Digital current loop module
103
Unregulated AC/DC adapter can be used, if the current is limited to 20 mA at
least by a serial resistor.
Note.The host computer can restrict the loop supply voltage that can be used;
see computer specifications.
The serial line structure is a serial interfaced chain (daisy chain). At one end
of the serial line there must be a HMP240 transmitter and at the other end a
line master. A branch line can be made with a junction box.
+
SINGLE LOOP WIRING
TWISTED PAIR WIRING
CURRENT
LIMIT
LOOP
SUPPLY
-
HOST COMPUTER
HMP 243 TYPE
TRANSMITTERS
1 TO N PIECES
+
DUAL LOOP WIRING
TWISTED PAIR WIRING
CURRENT
LIMIT
LOOP
SUPPLY
-
HOST COMPUTER
HMP 243 TYPE
TRANSMITTERS
1 TO N PIECES
+
NOTE : HALF DUPLEX DATA TRANSFER.
HMP 230 DOES NOT RECEIVE WHILE
TRANSMITTING VIA SERIAL PORT MODULE.
RX -
RX +
TX +
TX -
RX -
RX +
TX +
TX -
RX -
RX +
TX +
TX -
RX -
RX +
TX +
TX -
RX -
RX +
TX +
TX -
RX -
RX +
TX +
TX -
The digital current loop module has separate lines for transmitting and receiv-
ing. Both single loop wiring and dual loop wiring can be used (see figure).
With dual loop connection it is possible to have a few more transmitters on the
same loop pair. The wiring of single loop connection is more simple. Data
transmission is achieved by switching the loop current on and off.
Normally current flows through the loop(s) even when the HMP240 transmit-
ter is not on, so switching one transmitter off does not affect the other trans-
mitters 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