Vaisala HMP260 - Manual (Page 104)

HMP260 SERIES
Appendix 4: Digital current loop module
HMP260-U017en-1.5
The digital current loop module has separate lines for transmitting and receiving. Both
single loop wiring and dual loop wiring can be used (see figure). Dual loop connection
makes it 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 HMP260 transmitter is not
on, so switching one transmitter off does not affect the other transmitters 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 circuit 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. The module will
not break if it is mounted in the wrong direction or to wrong pins, but it does not work.
Reverse wiring of RX+ and RX- or TX+ and TX- does not affect the module.