Vaisala HMP240 - Manual (Page 106)

HMP240
Appendix 3: Digital current loop module
M210300en
104
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. If
mounted in the wrong direction or to the wrong pins, the module will not
break but it does not work. Reverse wiring of RX+ and RX- or TX+ and TX-
does not affect the module.
NETWORK CONFIGURATION
Single loop operation
Bi-directional data on one pair and galvanic isolation are the advantages of the
current loop. Single pair/dual pair use is configured through wiring (see figure
on previous page).
Supplying power from the same end to the loops prevents crossover voltages.
• Connect wires to the transmitter's serial connector.
• Check the wiring.
The following procedure must be repeated with all transmitters.
• Open the transmitter cover.
• Pull out the digital current loop module, if it is already mounted.
• Set the serial port of the terminal to 4800 baud, even parity, seven
data bits and one stop bit, full duplex (4800 E 7 1 FDX).
• The serial settings of the transmitter must also be 4800 E 7 1 FDX
and the transmitter must be in STOP mode. If these factory settings
have been changed, they must be returned. Connect the RS 232C port
of the terminal to connector X17 on the top of the main board and
switch the power on.
RX GND TX
X17