Vaisala HMP230 - Manual (Page 126)

HMP230 SERIES
Appendix 4: Digital current loop module
M210225en-B
120
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. A single loop connection has simpler wiring. Data
transmission is achieved by switching the loop current on and off.
Normally current flows through the loop(s) even when the HMP230 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
incorrectly, the voltage drop from RX+ to RX- or from TX+ to TX- is below 1
V and the transmitter does not work. Even then the current goes through the
loop and the other transmitters can be operated normally.
When the loop supply is current limited, the data lines can withstand short cir-
cuit to ground and to each other. They do not survive connection of supply
voltage to the data lines.
The module must be mounted on the main board in the right direction. It can
be mounted in the wrong direction or to the wrong pins without breaking the
module; it simply does not work then. Reverse wiring of RX+ and RX- or
TX+ and TX- does not affect the module.