2 Analogue inputs in series with one loop instrument.

[data74]

Hie
i'm trying to connect a 4-20mAtxmitter so that 2 seperate plc's can see it. The input modules are a AB flex I/O 1794-... and the other is a controllogix 1756.....don't have the exact catalogue numbers.Now i've connect them in series but on the controllogix data table i get a strange negative value. When i connect a pure 4-20mA i get a reasonable value...eg 50000. Where am i going wrong??

 

[dpc]

Polarity?

Inputs need to be isolated and not powered - or at least only one should be powered.



David Castor

http://www.cvoes.com

 

[itsmoked]

Often the PLC expects one side of its input to be ground or common. If you hook two together one of them gets confused.

As DPC sez you either need them to be floated inputs or 'isolated' or you need at least one of them to be and it needs to be electrically 'above' the one that demands a side being grounded.

If they both need a ground then you need to get a 4-20mA repeater:

Sensor - supply - repeater - two 4-20mA outputs -> one to each PLC.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[data74]

Ok thanks guys, will give it a go later today.

 

[StephenVB]

your analog device and power supply operates with a specific range of input loads. If you connect two inputs in series you may exceed this range. As previously noted each input has a resistance and the input module measures the voltage across the resistance to get the input value. With two resistors in series you may not have enough supply voltage.

 

[cjw32]

Check polarity, from item powering the loop is + to +, - to -, remainder of loop is + to -, - to + (if more than 2 items in the loop).

If the instrument is loop powered (2 wire) make sure only 1 PLC input card is configured to be active.

If instrument is powering the 4-20mA (4 wire) make sure both PLC's are configured to passive inputs.

It's unlikely that the input resistances will be exceeded. The item powering the loop will specify a maximum loop resistance, probably around 700 Ohms (if it's HART it'll also specify >250 Ohms). If it's passive, it'll specify a voltage range and a minimum loop resistance, probably >150 Ohms.