S7-300 Analog Input 4-20mA problem

Hello,

I have an: Analog input module SM 331; AI 8 x 12 bit;(6ES7331-7KF02-0AB0)

I have 4 channels that are set up for 2-wire 4-20mA signals. HW config is set and the jumper on the side of the module is set to "D".

The 4 channels are wired to a current source. When I hook a multimeter to the source, I see 20mA DC. When I hook a source up to a channel, it reads around 13mA. Even when the source is set to transmit 4mA, or anything in between, I always see 13mA. If I disconnect the source from the card, I read about 54mA out of the card.

Anybody know what I'm doing wrong?

Any idea about the difference in input resistance?

minerman said:

Hello,

I have an: Analog input module SM 331; AI 8 x 12 bit;(6ES7331-7KF02-0AB0)

I have 4 channels that are set up for 2-wire 4-20mA signals. HW config is set and the jumper on the side of the module is set to "D".

The 4 channels are wired to a current source. When I hook a multimeter to the source, I see 20mA DC. When I hook a source up to a channel, it reads around 13mA. Even when the source is set to transmit 4mA, or anything in between, I always see 13mA. If I disconnect the source from the card, I read about 54mA out of the card.

Anybody know what I'm doing wrong?

Click to expand...

Could be your meter is measuring something different.

CalG said:

Could be your meter is measuring something different.

Click to expand...

No, the raw data on the channel matches what I read with my meter connected in series.

I think I may have the wrong model of input module for this application? I have a 6ES7-3331-1KF02 and it is working for this application.

Are you using a self powered device? The 2 wire configuration will send power out to your device to power it. It sounds like you need to set the resistor cube to 4 wire which is for externally devices, like drives, etc.

RRobbins said:

Are you using a self powered device? The 2 wire configuration will send power out to your device to power it. It sounds like you need to set the resistor cube to 4 wire which is for externally devices, like drives, etc.

Click to expand...

exactly that, if your device send 4-20mA without being connected to the 2wire input, you have for sure a 4 wire transmitter (Self powered 2w is power and 2 other output signal) and your plc side set to 2 wires is looking for a passive sensor and send it a DC voltage to power it.

Dc voltage from sensor + dc voltage from your module create a comflic and read 13mA. You're lucky because in some case with cheap input module or wrong wiring, you could blew a channel up by creating a short...
In this case just switch the 331 module for 4 wires (Hard and software) and check youre wiring for this setting and enjoy

An easy way to protect your analog cards is to read the voltage from the card. If there is voltage present at the card and the wires from your device, you may burn one of them up. If you use 4 wire configuration on all analog cards, initially, you will save your cards, regardless of how the field device is configured. If the device does not power up, you should have a 2 wire device and will need to change the cube to give you power out. The 7KF02 cards are configured in CHANNELS of 2 inputs per cube X 4 cubes. The 7KF02 has the ability of mix and matching on the card. Most analog cards are totally 4 wire or 2 wire. If you have both, be careful, you may need 2 cards.