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RS 485 communication to/from stamp — Parallax Forums

RS 485 communication to/from stamp

Andy McLeodAndy McLeod Posts: 40
edited 2007-07-12 12:16 in General Discussion
I have a BS2pe and a beautiful Druck pressure transmitter - 16bit resolution over 20 bar. The Druck intercommunicates with modbus protocols through an RS 232/485 converter to any given terminal. What I would like to do is command and control/data log the pressure transmitter with the stamp. What approach can I use to create the 232/485 conversion in the smallest possible volume? I know there are any number of converters available (see http://www.bb-elec.com/productsubcat.asp?MainCategoryId=3&Trail=2&TrailType=Top ), but I want to create or buy a very tightly packed component because my final application is a handheld device about the size of a household flashlight.

Comments

  • FranklinFranklin Posts: 4,747
    edited 2007-07-12 00:55
    The BS2pe can talk RS232 directly. What do you need to convert? You also did not specify a model.

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    - Stephen
  • Andy McLeodAndy McLeod Posts: 40
    edited 2007-07-12 02:40
    The Druck model 33X http://www.kelleramerica.com/pdf/Series 33X - 35X.pdf outputs RS485. I need to convert this to RS232 for the stamp to read.
  • Mike GreenMike Green Posts: 23,101
    edited 2007-07-12 03:06
    There is an article on using RS485 in the Wikipedia: en.wikipedia.org/wiki/RS485.

    Note that RS485 is a signalling definition and doesn't define how the data is arranged. The document you indicated also doesn't give any information about Baud, data formatting, nor what RS485 signals are used other than the data lines.

    Maxim has a whole line of RS485 receivers, drivers, transceivers. Here's one for example: www.maxim-ic.com/quick_view2.cfm/qv_pk/5387
    Check out their application notes.
  • stamptrolstamptrol Posts: 1,731
    edited 2007-07-12 12:16
    Andy,

    The basic chip for RS485 work is the 75176. It is an 8-pin chip and handles one pair of '485 wires (half-duplex) and can both transmit and receive serial data at TTL levels which works perfectly with Stamps. New versions have better surge protection built in.

    As Mike pointed out, this will only address the the issue of signal levels.

    The MODBUS protocol has been around since the smoke cleared and can be a bit daunting to implement on the Stamp. The best proceedure is to use a MODBUS test program (Google it) to find out what the query sentence will look like, then store that sentence on the Stamp as DATA or in a SEROUT command.

    From your post, it looks like you can work at the RS232 level if the cable distances will not be too long or if you can run at low speeds. That will let you concentrate on getting the MODBUS side of things working.

    Cheers,

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    Tom Sisk

    http://www.siskconsult.com
    ·
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