Parallel RS232 connection needed

Discussion in 'The Projects Forum' started by CollieJim, Jul 2, 2015.

  1. CollieJim

    Thread Starter Member

    Nov 27, 2004
    25
    0
    I have a RAPS off-grid solar PV system using Outback Power Systems electronics. In the house I have their Mate 2 Remote Controller installed to keep track of it. The Mate 2 mainly displays status, but also can accept a limited number of commands to sent to the inverter/chargers.

    The Mate 2 has an RS232 port which I am using to connect to a greenHouse greenMonitor Lite computer for logging and a web interface.

    My project is to add a simple display powered by an Arduino Nano featuring State of Charge (SoC), battery voltage, and net current which can be seen across the room. I need to keep the current logging computer, so I need to be able to tap the RS232 connection.

    Is it feasible? I thought about just attaching wires to the Tx and ground pins but loading would probably be a problem.

    Thanks
     
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    2,913
    2,187
    A regular RS232 receiver is a high impedance input so a tap on the line with a short length (several feet) of wire won't be a problem if properly isolated.
    The ASCII protocol seems fairly simple: http://www.outbackpower.com/downloads/documents/mate/mate_serial_communicationsguide.pdf
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
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    Agree. You can't parallel up two transmitters, but one transmitter can drive multiple parallel receivers. How many depends on the age of the receiver chips. Newer CMOS chips present almost no load, while older bipolar chips probably can not be added on beyond 2 or 3 pieces.

    If you want long wire runs and guaranteed multi-tap performance, consider RS-232 to RS-422 converters. The RS-422 protocol is designed for this type of application.

    ak
     
  4. gmcmurry

    Member

    Mar 29, 2010
    11
    0
    I do this all the time. You can go 50 feet easily and at extremely high baud rates. My project uses 38.4K with multiple receivers. I sometimes think I might put a 1K resistor on the end of the wire as a termination, but so far, no problem. I use MAX232 line receivers. They seem to do will.

    Greg
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If you look at the dv/dt of a MAX232 output, you can calculate that a termination is not necessary for such a short cable length.

    ak
     
  6. CollieJim

    Thread Starter Member

    Nov 27, 2004
    25
    0
    Thanks. I looks like I can proceed. According to the spec sheet for the ATmega328P the pins can sink an absolute maximum of 40ma. Without knowing what the effective load would be I will try the connection with a 10k resistor.

    I've had the Communications Guide for a while. They don't make identifying the MX and FX records easy. Without the logging I would have had to trace the wiring to determine port numbers.
     
  7. CollieJim

    Thread Starter Member

    Nov 27, 2004
    25
    0
    I have the connection working, but my Saleae Logic was giving me lots of rubbish and framing errors until I told it to invert the signal. Then I saw good records for both inverters, both chargers and the FLEXnet, with a full set arriving every second.

    I've not heard of inverted serial before.
     
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