Emerson Exchange 365

Single Ended and Differential Ended Analog Input

Hello,

Based on My study on M-Series and S-Series analog input cards (4-20mA) i learn that it is all differential input kind of circuit. 

if we compare to PLC then i have notice that PLC has Single Ended and Differential Input kind of wiring.

can you advise if my understanding above is correct ?

Also i learn that M-Series and S-Series Analog Input Cards use 24V DC from bussed power for powering the 2 wire transmitter loop ? is there any way we can use external power supply to loop power 2 wire transmitter connected to m-series and s-series analog input cards ?

Thanks for all your help.

Regards,

Mitesh Maniar

Mitesh Maniar

• 20 Mar 2018 2:53 PM
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4 Replies

• Andre Dicaire

  

  • 20 Mar 2018 6:31 PM

  First, the Analog Inputs of M-series and S-series are Single ended, not differential. The simplified circuit diagrams in Books On Line for the AI 4-20 mA channels all show that the 8 or 16 inputs share the same DC Common.
  
  A differential input would be floating, or isolated. The 0-10 V AI card is an example of a differential input.
  
  The M-series 8 channel card can be used with the 4-wire Terminal block, which exposes the common references for each channel. This allows a 4 wire device to power a 4-20 mA isolated signal to the AI Channel. The signal is grounded at the DeltaV card so the transmitter's 4-20 mA output must be isolated.
  
  The S-series 8 Channel Terminal block has three terminals per channel: 24 V DC, Signal and Common. You can use this to connect a two wire or a 4-wire signal to any channel. Again, signals must be isolated in the field as they are referenced to Ground in the AI Card.
  
  If you are marshaling the field wiring to a terminal strip, with cross wiring into the DeltaV IO Card terminals, you can have signal common terminals in the terminal strip, and connected these to DC common, and then pass the signal wire to the AI terminal block's Signal terminal on the 2 wire terminal, and power the transmitter with separate power signals. However, this leaves the internal signal wire unshielded. This could interfere with HART communications. If the device is HART, you should connect the signal pair directly to the right type of terminal block so that the shield protects the signal all the way to the IO Terminals block.
  
  If you want to use an external power supply to power 2-wire devices, and connect these to the DeltaV AI Channels, you could do this. Run 24VDC to a fused terminal, with adjacent signal return terminal. Connect field device to these two terminals, and wire the signal on to the Signal terminal of the AI card. Again, you end up exposing the unshielded single signal wire to noise. The twisting of the two wires is an important protection against noise.
  
  The DeltaV AI channels provide current limiting within each channel that prevents damage to wiring. If wires are shorted, the AI card detects a Short Circuit condition and current is limited so no damage is done, no fuse is blown. When wire fault is corrected, signal returns. With a fuse, a short circuit condition is immediately replaced by an open loop condition as the fuse blows. And you have to replace the fuse.
  
  The current limit in the channel power causes a voltage drop internal to the card proportional to the signal. So at 20 mA, the signal drops 5 volts on the channel load resistor and a couple of volts on the current limiting circuit. The available voltage at the transmitter at 20mA can be limited to about 16 volts. If you have transmitters that need more than this for full operation, the Buss Power 24 VDC power supply can be increased (no more than 26 VDC) to provide extra voltage across the transmitter at full power. Alternatively, you can externally power the transmitter for maximum voltage to the transmitter. Most 2 wire transmitters operate well below 15 Volts at full signal. If there is excessive voltage loss on long small gauge wiring, the added voltage may be needed to overcome this.
  
  Unless these abnormal conditions exist, you should simply use the Bussed Field power through the AI card and avoid blowing fuses.

  Andre Dicaire

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• Mitesh Maniar

  

  • 21 Mar 2018 6:12 AM

  In reply to Andre Dicaire:

  Thank you Andre for Detail Answer on my question. it clarifies on my understanding.
  
  Actually i was comparing it with PLC5 Card wiring and so confuse between Single Ended/Differential Input due to extra terminals on PLC5/Control Logix Analog Input Cards.
  
  I noted your advise to use Bussed Field Power trough AI Card but what incase if we may face condition where we need to replace card like 1771-IFE (PLC5 Analog Input Card) which always use external power supply to power the 2 wire loop powered transmitter. i was just thinking easiest way of using that external power supply instead of delta v 24V DC bussed power.
  
  or can i connect that external DC power which is used for powering 2 wiring transmitters loop to 24V DC bussed Power + & - ?
  
  or any suggestion to handle such condition ?
  
  Also if we use external power supply to power the 2 wire loop powered transmitter then do we need to leave 24V DC Bussed Field Power connection disconnected ? if we connect that then i believe it will be clash between two separate 24V DC Power from bussed and external power supply ?
  
  Hope i am able to explain my question. please let me know if it is not clear.
  
  Once again thanks for all your help.
  
  Regards,
  Mitesh Maniar

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• Andre Dicaire

  

  • 21 Mar 2018 4:38 PM

  In reply to Mitesh Maniar:

  Retro fitting an existing site and displacing the incumbent system always brings unique challenges. I can't tell you what the right solution is for your case. Installing the new system according to the old system's limitations or requirements means you are not installing the DeltaV system according to DeltaV best practices. That's a decision you have to make.
  
  AS for connecting the DC Power, you have to respect the DeltaV power and Grounding requirements, which means the DC Power used for field devices must be grounded consistently with DeltaV requirements. Not connecting this to the 24 VDC Bussed Power still requires that this 24 VDC source be grounded appropriately, i.e. to the DeltaV DC Common.
  
  You need to review the wiring diagrams for the DeltaV cards and understand their normal recommended installation requirements. Doing so, you will notice that the AI 4-20 mA cards are connected to DeltaV DC common through the bussed field power connection. So you must connect this common to the 24 VDC common to ensure proper operation of the card. The DC Common must be properly grounded, as well as the DeltaV Carriers for their 12 VDC power to the IO Cards.
  
  If you had reviewed the DeltaV IO card wiring schematics in Books on Line and understood how the standard installation works, you should not be asking about whether you need to connect the bussed Power common connection. I think you need to engage some services to get some expertise on your team.
  
  As I mentioned in a previous post, deviating from the recommended installation of DeltaV field wiring may have negative impact on HART communications. Proper grounding and shielding of the IO subsystem and field wiring ensures reliable HART communication. The 4-20 mA control signal can handle noise conditions but HART communication can be lost. Since PLC5 IO likely did not support HART this was not a concern. But if you are expecting to leverage existing HART device information through the DeltaV Plantweb architecture, taking shortcuts on the field wiring/power may cause you issues with HART.
  
  Please get help from you Emerson service provider to review your migration plan.

  Andre Dicaire

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• Mitesh Maniar

  

  • 22 Mar 2018 4:28 AM

  In reply to Andre Dicaire:

  Thank you Andre for all your help answering my question.

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