Flex I/O won't power loop device

I have a Flex I/O IE8H card and it will not power a loop powered Siemens level transmitter. I can simulate with my process meter just fine but when I hook up the level transmitter it won't even power up the display. I have a second transmitter that I hooked up right in the PLC cabinet to eliminate any field wiring issues. Anyone have any ideas?

Flex I/O does not supply loop power. You will have to use an external power supply. You could power the loop from the terminal base, assuming you are using the TB3 base which should have +24 and Common on the lower two terminal rows. You would need to send +24 to the transmitter + terminal, wire the transmitter - terminal to the + input on the Flex, then wire the Flex - to 24VDC common.

There are only a few Allen Bradley analog input modules that supply loop power from the input terminals.

The card is a Hart card and uses a TB3G base. The wiring diagram from AB shows landing a two wire device with + on terminal 0 and signal on terminal 1. I do have the base fed with +24v and -24v on the appropriate terminals and power light is on the card. When I set my 787 to simulate and connect + to terminal 0 and - to terminal 1 I can simulate a milliamp signal that is seen by the PLC but when I connect the Siemens Probe LU it doesn't even power up the screen let alone generate any signal.

Sorry, I've only worked with the IF8IH Flex HART module. Check knowledgebase article 50734. The IE8H only supplies a maximum of 23.7 volts to the transmitter. At 4mA, the voltage would be less. At 20mA, the voltage is just above 17 volts. What are the voltage requirements for the transmitter?

That very well could be the issue. Good point.

According to the Siemens LU technical spec, the minimum operating voltage is 17.8 volts at 300 ohms loop resistance. The IE8H module is a 273 ohm load. I believe the voltage drops to an unacceptable level when the LU is trying to power up.

Is your device a Siemens Probe LU, the 2 wire ultrasonic level transmitter?

I suspect that Flex I/O is designed Intrinsically Safe and it limits current to the field device. This is the 3rd time I've heard of Flex I/O (or P&F's version) not powering up Siemens Probe LUs.

Ultrasonics are power hogs requiring a long charging period of current with which to ping the transducer. If the current or voltage is limited the Probe never intializes.

You might check what the voltage is and see if the Probe is getting 17.8V minimum.

I agree Dan. The supply is indeed intrinsically safe on these modules. We used the LU Probe a lot in a place I used to contract to and I remember similar issues. We always provided a separate loop power supply.

danny.michael said:

According to the Siemens LU technical spec, the minimum operating voltage is 17.8 volts at 300 ohms loop resistance...

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Danny, where did you get those specs from?

Lance Johnson said:

...when I connect the Siemens Probe LU it doesn't even power up...

Click to expand...

Our friend seems to be using the Siemens Sitrans Probe LU. This is its data sheet. According to this, the nominal voltage should be 24VDC @ 550Ω max., up to a max. 30VDC. So the Probe LU's working supply voltage range is 24VDC-30VDC. The 1794-IE8H provides the necessary HART minimum resistance of 250Ω (273Ω), which is also under the 550Ω. But the modules supply starts at 23.7VDC @ 0mA and drops to nearly 17VDC @ 22mA. The transmitter is 4-20mA, not 0-20mA, so when trying to power up, if the transmitter is at 4mA, the voltage will already be somewhat lower than 23.7VDC. If at 20mA, or above, forget it.

This is from the Flex I/O Selection Guide - Publication 1794-SG002D-EN-P

Analog Input Modules require supplied 24V DC (19.2…31.2V DC) and consume the currents listed in the module specifications (>17V @ 22 mA, >23V @ 0 mA). In addition, adequate power must be supplied to the 1794-IE8 and 1794-IF4I transmitters to deliver input terminal voltage or drive 20 mA into the input impedance listed. This power source may be the same as the module power and can be included in the power supply calculated.

I would look to put a separate supply on the loop for this passive transmitter, even if temporarily, just to test the theory. Flex offers the 1794-PS13 (1.3A) or 1794-PS3 (3A) PSUs, but any compatible PSU will do.

Regards,
George

I had suspected the flex card didn't have the output to power the LU Probe. As a quick test I connected the unit + terminal directly to the +24v from the power supply in the cabinet also powering the flex cards and connected the unit - terminal to terminal 1 on the flex card terminal (signal) and this did not power the unit either. I thought I could do that as a work around, any other ideas?

George - the info came from Siemens manual A5E32337695 at https://a248.e.akamai.net/cache.aut...E4MTMzAAAA_19101050_HB/A5E32337695_001_AA.pdf. The voltage info came from the graph on page 91. I just noticed on page 88 a note that states the unit will not start reliably with a power supply that limits the current to less than 25mA.

Ahh ok Danny. I had read the same info this morning. I was writing this and then saw your reply. I'll post the similar info anyway...

From the latest Siemens Sitrans Probe LU (HART) Instruction Manual...

Page 88 - Startup Behavior Notes:
SITRANS Probe LU (HART) is designed to start reliably with a power supply capable of delivering at least 25 mA. When connected to a power supply with a current limit of < 25 mA, the ProbeLU may not start reliably

If you look at the chart on Page 88 it shows the start up current draw characteristics from an intrinsically safe current limited power supply of 25mA. It draws 25mA for the first 3secs, then drops to around 23.5mA for the next 8secs, and finally settles around the 17.8mA mark.

The 1794-IE8H is intrisically safe, so it is limited to a 22mA output current, which does not meet the minimum 25mA required for the Probe LU.

G.

I don't have details on the wiring, but a colleague said that he got Probe LUs to initialize and operate by using an external loop power supply with the I/S version Flex I/O, which was feasible because the area was a general purpose area, not a hazardous area.

Lance Johnson said:

I had suspected the flex card didn't have the output to power the LU Probe. As a quick test I connected the unit + terminal directly to the +24v from the power supply in the cabinet also powering the flex cards and connected the unit - terminal to terminal 1 on the flex card terminal (signal) and this did not power the unit either. I thought I could do that as a work around, any other ideas?

Click to expand...

Lance, I missed this the first time I read it.

HART can only poll the transmitter if there is a sufficient impedance in the loop between the power supply and the transmitter. It works using digital AC signals imposed on the current loop. Without impedance, power supply capacitors act like a low pass filter canceling out these AC signals.

Terminal 2 on the 1794-IE8H provides termination for the + wire to the transmitter. The internal 273Ω resistor is in series on this wire. When you moved the transmitter's + wire from 2 directly to the + 24VDC terminal of your cabinet PSU, you effectively removed the necessary impedance. You would have to place a minimum 250Ω resistor in series with the PSU and the + wire to the transmitter for HART to communicate properly. I'm not sure if wiring it that way will work, or not, but you will definitely need the impedance in the loop for the HART module to read in the data.

As Dan has mentioned it, so should I. We don't know if your setup is for a hazardous area or not. If it is, then bypassing the Intrinsically Safe limited PSU of the 1794-IE8H with an unlimited PSU would be a no no. You would have to get a PSU that is intrinsically safe, but only current limited to 25mA. Or, replace the Probe LU with another compatible transmitter.

Regards,
George