Signaling line circuit

Discussion in 'The Projects Forum' started by RahulK, Sep 25, 2015.

  1. RahulK

    Thread Starter New Member

    Jul 10, 2015
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    How to communicate over a 24V data powered DC line?
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    AC coupling
     
  3. RahulK

    Thread Starter New Member

    Jul 10, 2015
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    I have to design a system which connects with n number of devices (Input/output devices such as sensor, beam detector and etc).

    All these devices are 24V DC operated including the main system.

    I want my system to communicate with these device using the voltage level. I want to communicate using the voltage level. in asynchronous mode using the 24V DC power cable. I am planning to have a voltage level between 18 (Logic 0) and 20 (Logic 1) as protocol voltage.

    I have understood a few things but I still don't know how to start.

    For ex. 100 sensors/devices will be connected in parallel with 24V DC line. Each of these device will have a micro-controller operating at 5 volt, each device will also have their address. Now, I want to communicate with every device on the line, every device should send its address (for ex. 01010101) within the protocol's voltage level which is 18V DC for LOGIC 0 and 20V DC for LOGIC 1. I know I have to do an asynchronous mode of communication which will have the following data package [START BIT] [8 DATA BITS or Address of the device] [PARITY BIT] [STOP BIT].

    First of all, I want to do it for a single device and system. In this, the communication will be started only by master, slave devices should only interpret the commands and reply in the answer window formed by the MASTER SYSTEM. This answer window will have its own voltage level, it could be any voltage between 22 to 23.

    I know what I have to do, but I don't know how to start. I will be using ADC0831 for this purpose.

    I sincerely request all the respected members here to help design my protocol, like, what should be the ideal transmission method I adopt here.
    Should I make 18V as logic 0 and 20 V as logic 1, to interpret the data. Or should I do something different, like 001 as logic 0 and 011 as logic 1. I really need help.

    Actually, I need to develop a protocol for this. My end product is an intelligent fire alarm system. I've know how conventional system works, both at hardware and software level. In conventional system, you check zone line voltage and trigger and alarm if the voltage is down to a defined level, so you don't monitor the every sensor connected in the zone line but you only check the zone line. In intelligent system I need to check not only the zone line voltage but also every device for its address, so that main panel can display the precise location of the device. I am a programmer so my knowledge about electronics is limited. While searching the web I found several protocols such as FlashScan, RS232.

    Please also see the attached file.

    Regards,
    RahulK
     
    Last edited: Sep 25, 2015
  4. Papabravo

    Expert

    Feb 24, 2006
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    Instead of designing something new, why not use an existing industrial protocol such as DeviceNet. The 5-conductor cable is robust, has a power pair, a data pair, and a shield. The underlying CAN protocol is robust and fault tolerant. You could roll your own with far inferior results as opposed to getting to market quickly.

    https://en.wikipedia.org/wiki/DeviceNet

    One more thing. Putting data on the power pair is really really bad idea, if any of the devices have switching regulators. Input transients can drive a cheap (inexpensive) SMPS absolutely bats**t crazy.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you are required to use two-wire alarm cable, and cannot replace all of it with 5-wire cable as suggested above, then bidirectional data communications can be done, but it is not a simple task.

    Using a DC power line to send digital data in one direction, like going from a central control panel out to a long string of sensors, is relatively easy. Basically you turn the power supply voltage up and down for the data bits as you are suggesting. But that works well because the power supply has a very low output impedance. For the individual sensors to modulate their data on that power line to send it back to the central controller is more difficult because the power supply regulator is trying to hold its output constant.

    One way around this is to put the data on a high frequency carrier on the DC power line. This is how the X-10 and other home automation systems communicate over the AC power lines. Another way is to change the system power supply to a constant voltage with a medium output impedance, something like 200 ohms. This is how intercoms work in TV studios. Either way, there is going to be some circuit design involved.

    ak
     
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  6. Papabravo

    Expert

    Feb 24, 2006
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    Which gets back to my original suggestion: AC coupling. I would limit the signal to about 200 mV P-P. Your choice of carrier frequency will also be critical since you can't control the impedance of alarm cable and it is unlikely to be shielded. If there are SMPS connected to the cable, your data is going to look like an event the control loop needs to respond to. If you are not very careful with this design you will drive some of them nuts trying to follow the input voltage transients. It sounds like this project is quite beyond your capabilities. If this is for your work I strongly suggest that you find expert advice.
     
  7. RahulK

    Thread Starter New Member

    Jul 10, 2015
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    Thank you everyone for valuable replies.

    I can't use more than 2 wires to develop this system.
    One thing is clear now that the loop line voltage is the limit for slave system. Slave can modulate within the maximum loop line voltage but not above the loop line voltage. That means, system can choose any voltage level, say 20 V for 1 and 18 V for 0, to poll the slaves in the loop line, and the upper limit for slave is 20 V. I will be polling the slave devices by generating a square wave rather keeping the line voltage an ideal DC wave which is a straight line. Slave can't modulate any voltage above 20 V since it is the upper limit for it, but it can pull the voltage down to some level.

    Now, say, after every poll I pull the loop voltage down to, say, 16 V and let the slave reply within this range. 16 V will be my answer window using which the slave will reply. If I take 16 V as logic 1 and, say, 15.5 V as logic 0 or the vice-versa and read these levels in the master with an ADC, what are the possibilities then? I understand that some serious circuit designing will be required for that.

    My question is that, can it be possible to do that within the millivolt levels while we know that the power supply is trying to keep the loop line voltage constant, this time the constant voltage will be the upper limit of answer window.

    It may be beyond my capabilities. But with few valuable insights I will make it happen.

    Please also see the attached file.

    Regards,
    RahulK
     
  8. Papabravo

    Expert

    Feb 24, 2006
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    I wish you the best of luck in your enterprise.
     
  9. RahulK

    Thread Starter New Member

    Jul 10, 2015
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    Thank you,

    Regards,
    RahulK
     
  10. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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