prop communication and pc connexion ?

Hello,

I have a parallax propeller for years but I am not a specialist. And I apologize for my English, I'm French. In short, I try to communicate several propeller together (not necessarily just two). And ideally with other platforms (pc, arduino, ...). Which protocol / connection do you recommend? I looked on the ethernet side (see ethercat) (it would be convenient to have a single network socket and use a switch) but it seems complex without a dedicated external component (ideally I would not want any other component and use only one cog). Without having to make a connection it would probably be better (like UDP). I want to be able to exchange short messages (like 32 bits) at low speeds. A transmitter and all receivers. No wireless. What would you use? THANK YOU

Comments

  • jmgjmg Posts: 13,788
    You should define 'low speeds' and 'several', and what distances between units ?
    The simplest link will be UART Async Serial, and for modest/local lengths TTL-Serial is ok.
    For higher distances, you could look at RS485 links, which are just better differential line drivers/buffers, and direction control allowing multiple nodes from one master.

    There are many USB-UART (TTL) modules, and fewer with RS485 drivers, but I see
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    (etc) look suitable.
  • On the propeller side and other Microcontrollers serial transmission is most common.

    If you just need one transmitter and everybody else is just receiving you can basically tie TX from your transmitter to RX of all receivers.

    All receivers will receive the same data and you would need to make a transmission 'packet' with a address followed by you data, so the receiver knows that the data is for himself.

    What you have to take care of are voltage differences, the Propeller is 3.3 volt the Arduino usually 5 volt. But you can put a resistor between the 5 volt transmission line and the propeller pin to take care of that.

    On the propeller side I would advise you to leave pins 30/31 for USB and programming and chose another pin for receiving, you can use serial on any pin.

    Enjoy!

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  • Have you considered an RS-485 network? They're easy to implement and dealing with RS-458 is easy with any micro. You will need to create your own messages, but that will be part of the fun.
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  • Peter JakackiPeter Jakacki Posts: 8,517
    edited 2019-06-26 - 03:16:12
    If you already have a RS-232 serial port on your PC then you can simply run the transmit line out to all the Props and simply use a 100k resistor on the input of each Prop. Even if the RS-232 port were to output +/-12V (which they rarely do) the 100k resistor is all that is necessary to limit the current. The serial receive routine would simply read the data as inverted from TTL. If you use a USB to serial converter then it doesn't matter that it outputs 0..3.3V or RS-232 levels since the Prop will always trip around the 1.65V level which is pretty much what "proper" RS232 receive chips do anyway.

    Since it is only 32-bits I'm guessing the speed needn't be all that high and the only concern with a lower baud-rate is the latency from when you send the message until it fully received which improves by increasing the baud-rate. If all this is inconsequential then just use a low baud rate and send the 32-bits as ASCII HEX if you like including a terminator just so it is easy to debug. So $CAFEBABE<cr> with a $ sign for message "preamble" is one example.

    BTW, some will argue that you need RS-485/422 for long runs but if your long run is only 10's of meters and relatively noise free then there is nothing wrong with this scheme. Many will connect fast 1-wire 3.3V networks over 100's of feet without too much of a problem and TTL or RS232 serial is no different. If you need RS485 then that is also very simple but each Prop will need a RS485 or RS422 receiver although a comparator + resistors is way cheaper.

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  • For a multi(master)-node communication I think the simplest, easiest and more reliable solution is RS485.

    I usually use Maxim's MAX3160E(where unsure)/MAX3161E(where sure), a transceiver whic can pair a single ttl serial port to RS232 or RS485/422 lines.
    With the MAX3160E I usually provide a single DB9 (or RJ45 for PCB space saving) connector while with MAX3161E I use both a DB9 for RS232 and 2 RJ45 (or 4 pin screw terminal) for RS485/422.

    But as you mentioned also Ethernet have also a look over this converters which are now my first choice and I use them also to update Microchip PIC and Parallax P1 firmwares from remote locations: EM510 or EM2000 (EM2001) both with or without WiFi/BLE addon
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  • Hello,
    It was late last night (in France) so I realize that my message was not very clear ... Thank you for your answers in any case. Some precisions.

    I want to communicate several P1, like a bus (no routing and point to point). All can send to everyone (and everyone sort of what concerns him). That's why I talked about ethernet and switch. It is also necessary that a computer can be connected to read and to emit also (without special privilege).

    Ideally I would like the least component possible in addition to the P1. Except for connecting them (switch type) or to connect / convert the PC. Ideally a modern PC without serial (or parallel) output ...

    I know RS232, RS485, etc. But I only used them between 2 terminals. You can connect x machines without having to multiply connections (I understand 2 ring for example but if you want a single plug?).

    THANK YOU
  • When you say "all can send to everyone" that means that anyone of them can send and others must listen and be quiet. That implies that the connection needs to be half-duplex and multi-master?

    Still how far apart are these units? Is it really multi-master or is the PC the communications master?


    Many modern PCs have a serial port for the reason that unlike USB serial, there is no latency and no special drivers required, but I just mentioned this previously to show that a solution could be that simple. However I am unsure of your requirements but if it were in close proximity then a single unbalanced half-duplex line would suffice with a simple circuit on the PC end.

    Here is a video of multiple Propellers connected together over RS485 and although the Props are bunched together they could just as easily be 100's of meters apart if need be.

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    --->CLICK THE LOGO for more links<---
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  • henrib75 wrote: »
    Hello,
    It was late last night (in France) so I realize that my message was not very clear ... Thank you for your answers in any case. Some precisions.

    I want to communicate several P1, like a bus (no routing and point to point). All can send to everyone (and everyone sort of what concerns him). That's why I talked about ethernet and switch. It is also necessary that a computer can be connected to read and to emit also (without special privilege).

    Ideally I would like the least component possible in addition to the P1. Except for connecting them (switch type) or to connect / convert the PC. Ideally a modern PC without serial (or parallel) output ...

    I know RS232, RS485, etc. But I only used them between 2 terminals. You can connect x machines without having to multiply connections (I understand 2 ring for example but if you want a single plug?).

    THANK YOU

    How about one pullup resistor for the bus and one current limiting safety resistor for each P1 pin that connects to the bus? Carrier Sense Multiple Access and Collision Detection code required, but not too hard to do. Down side is it needs 2 pins. Another option is to use a token passing scheme.
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  • jmgjmg Posts: 13,788
    henrib75 wrote: »
    I know RS232, RS485, etc. But I only used them between 2 terminals. You can connect x machines without having to multiply connections (I understand 2 ring for example but if you want a single plug?).
    Yes, the RS485 is designed for multiple nodes on a parallel 2 wire bus.

    henrib75 wrote: »
    I want to communicate several P1, like a bus (no routing and point to point). All can send to everyone (and everyone sort of what concerns him). That's why I talked about ethernet and switch.
    It is also necessary that a computer can be connected to read and to emit also (without special privilege).

    That gets a bit more complex, as usually someone in these connected nodes, acts as a master.
    With no master, you need to detect collisions, and retry if not the same, with some random wait period..

    A P1 can detect and release inside a single bit, but a computer cannot do the same.

    The CAN bus transceivers are better at collision detect than RS485 designs are, however you can re-wire a RS485 so the TX enables the driver, and pre-bias the RS485 bus, to get to 'CAN-like' operation.

    The board I linked above looks like it can do that, via their nifty BUS SP339 driver which has RS232/RS485/RS422 in 8 possible modes.
    https://www.maxlinear.com/document?id=21620&languageid=1033&type=Schematics&partnumber=XR21B1420
    You would bridge TXD to DIR1 and use INVDIR pin to have drive-enable active-low.
    The PC can send short packets, and check if the address-part echos undamaged. If yes, and your echo is larger than your send, the appended data is the reply.

    In this case the PC is interrogate originator, it is probably not nimble enough to be interrogate immediate echo. (maybe if your rate is slow enough ?)
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