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uC + DAC + U/I + PLC Beckhoff KL3454

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kedzior87

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uC + DAC + U/I XTR117 + PLC Beckhoff KL3454

Hi !
Sorry for my english.
I'm trying to send 4 analog values (water level) from microcontroler (atmega32 or atmega128 ) to PLC ( Beckhoff PLC ). For many reasons I can't use serial communication (RS232 or others). The values must by send to analog input terminal "Beckhoff KL3454" :BECKHOFF New Automation Technology
.
I've decided to use 4 D/AC's : DAC121C085
DAC121C085 - 12-Bit Micro Power Digital-to-Analog Converter with an I2C-Compatible Interface and External Reference
connected to atmega via I2C bus.
D/AC's output is 0-5V.
Output's of each DAC are connected to four XTR117 - 2 wire current loop :
http://focus.ti.com/docs/prod/productfolder.jhtml?genericPartNumber=XTR117
The output of XTR117 if directly connected to analog channel in KL3454 as shown in picture below:
6080612600_1299196787.jpg
R1=125kOhm
R2=31,25kOhm
I'v created a simple board with one DAC and one XTR :
Zdjęcie0444.jpg
and here starts my problems :-(
DAC works fine ... i can control DAC output voltage through i2c bus and it really works.
But when I connect the output of XTR to PLC analog input channel nothing happens, only transistor gets warm.
The KL3454 control led of this channel is always red ... (means error).

When I connect both channel pins ( +24V and "channel in") throuh resistor (for example about 5kOhm or potentiometer[used as controled resistor] everything works fine)
so could anyone help me with my problem?
Maybe it's in connection ? maybe KL3454 will not work with XTR117. Maybe there should be a capacitor beetwen channel pins Or maybe anyone knows better way to send analog value from uC to analog (current) input of PLC.
 
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