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minhdam06102

analog signal to 2 module

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Dear Mr/Ms, I have an ultrasonic sensor with 4-20mA analog signal output. But I have 2 control panel with 2 PLC separate, two PLC have analog input module . Can I send signal for 2 analog module of 2 PLC with one sensor? If can how can I make them - Analog signal will connect analog (+) and analog (-)with module analog of 1st PLC and then from that terminal (analog terminal of 1st PLC) wiring to analog (+) and analog (-) of 2nd PLC ? or - Analog signal from sensor (+) will connect with (+) analog 1st PLC then (-) analog of 1st PLC will connect with (+) of 2nd PLC then (-) 2nd PLC will connect with (-) sensor ? or - Analog signal from sensor (+) will connect with (-) analog 1st PLC then (+) analog of 1st PLC will connect with (-) of 2nd PLC then (+) 2nd PLC will connect with (-) sensor ? Would you please help me to solve this? Thanks & best regards,

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1) What type of PLC/analog module? 2) Usually it is realized with data exchange between two PLCs.

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Dear Mr, We use mitsubishi Q series Q02H, analog module is Q68ADI. for 2 panel is the same type but 2PLC not comment 0VDC and 24VDC. But if we not use analog separate( 1in to 2out) accessories. We just want to connect by wiring to reduce the cost. Would you please tell me how to do that. Thanks & best regards,

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Sorry, there is no way to reduce the cost: 1) To connect one 4-20mA analog signal to both PLCs simultaneously would be possible, if you use analog modules with isolated channels (modules with N-suffix). Otherwise the PLCs may be destroyed. Thus to use this way you have to replace the Q68ADI to Q68ADIN on both PLCs. 2) To connect the analog signal to one PLC and then to transfer the measured value to another via some communication channel requires a presence of communication modules on both, that will be too expensive.

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I stand to be corrected on this, and I haven't tried it, but I would have thought it would be possible with two Q68ADI channels on 2 different PLCs. I've attached a graphic of how I think it would work with both active and passive types of transmitter. As far as I'm aware, the ADI channels pass the mA current through a high spec resister and measure the voltage drop across the resistor, effectively calculating the voltage drop using Ohm's Law. As long as the sum of the two resistor is less than the load that the transmitter, then both channels should be able to see the current in the loop?

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yes, and - as long as at least one of them has floating input (not referenced to AGND or 0VDC)

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@Colin Carpenter, @panic mode Do not forget that Q68ADI is 8-channel module and for sure besides the signal, which topicstarter want to duplicate onto both PLCs, other analog circuits are also wired to these modules... Edited by Inntele

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I'm a little confused regarding the references to "grounds, references and floating" with respect to other channels wired to the Q68ADI. I've installed lots of these cards and have: 1. Wired 4 channels from two active transmitters onto a card with 4 other passive transmitters. 2. Tested channels with a handheld (battery powered) milliamp injector while other channels are operating on a mix of passive and active signals. 3. Shared a 4-20 mA signal from a passive transmitter between a Q68ADI channel and a Honeywell chart recorder by wiring the two analogue inputs in series. 4. Inserted a multimeter in series with a Q68ADI channel to check the mA flowing. and all without problem. I've attached the wiring diagram from the Q68ADI manual and I can't quite see why each channel cannot be treated as a separate entity. Surely, as long as the mA flowing through each of the individual 250 ohm resistors is happening ( ie the mA circuit is made) then the Q68ADI channel only measures the difference in voltage between the two ends of the resistor, and, as the voltage difference is not reliant on having a ground to reference it to, it all seems to work fine. I fully respect your experience and opinions on this as it seems that I may be missing something ........ it's just that my experience seems to suggest that these things just work.

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Dear Colin, On the schematic diagram of one channel of Q68ADI module are present two resistors, connected to AGND. AGND circuit is common for all 8 channels, thus all input amplifiers of the module are connected with each other and referenced to AGND. To connect a channel of one module with a channel of another in serial is possible only if the 4-20ma output of sensor is supplied from a separate power source, that is not referenced to other analog circuits of both modules. Edited by Inntele

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Please could you define what you mean by a power supply that is referenced to other analog circuits of both modules. I'm a little confused by that. Whenever we build a PLC cabinet, we never use the 24VDC capability of the power supply on the PLC rack, but instead use a larger, separate PSU. The 0 VDC side of this terminal is always connected to the earth of the chassis which is connected to the site earth. In the past, when we have wanted to share a digital 24 VDC signal between PLCs, we generally run a cable between the the two 0 VDC terminals and both PLCs can then see the input, though you could argue that as both PLCs have their 0 VDC connected to earth, this cable might not be necessary.

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Dear Colin, This is my comment to the comment, you deleted. Yes, it's possible to do and not only for active output, but for passive too, if the output is supplied from separate PS. However it's very dangerous action, which may cause to damage the PLC. That's why I not recommend strongly to do it. Once, I've seen the results of money saving - the PLC, which cost of several thousands euros, was thrown into the trash. The death came thru the analog input. The topicstarter may get two corpses and with Q-series the money losses may be in several times more, that accordingly may put a cross on his carreer.

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For discrete signal is possible to use this way, because the inputs are isolated by optocouplers. While for analog it gives a very big risk! The idea to use a ground as a conductor for small signals is not good. Moreover, an additional earth wire for signals is specially recommended to use to align ground potentials and to decrease a parasitic capacitance. Can say, we had a site, where we has been forced to install resistive shunts of 1,6k to each discrete input, to achive a 20mA current in wires, otherwise we'd just get false signals. The electric field at this site was so high that signalling LED lamps lit permanently without any visible voltage...

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