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RS-232 connection

Wenjun Huang
Expert 1770 points

I have 2 processors and 2 RS-232 devices (slaves). I would like to connect the 2 RS-232 devices to each processor through RS-232 protocol. At any one time, only one processor is active. I would like the active processor to access both the RS-232 device.

Is it possible to use just 1 RS232 interface on the processor to connect to the 2 RS-232 devices?

If it is not possible, I would be able to allocate 2 RS-232 interfaces for the processor. Each RS-232 interfaces on the processor will  be connected to each RS-232 device. Another question is that the one RS-232 interface on the RS-232 device, can it be used for connecting to both processors? Is there a RS232 multiplexer chip to route the RS-232 signal from the RS-232 device to the active processor?

  • 0
    TI__Guru**** 165236 points

    Wenjun,

    If the RS232 device has enough receivers and drivers for two RS232 ports then only one RS232 interface chip is required.
    If the processors allow voltage on it's I/O lines when it is disabled then both processors can be connected at the same time.

    Be sure that the driver logic inputs never float (DIN or TIN). A pull-up resistor can be used to prevent this possibility.

    Regards,
    Ron Michallick

     

  • 0 in reply to Ron Michallick
    Expert 1770 points

    Hi Ron,

    Thanks for your reply. What do you mean by RS232 interface chip? Is it a device that would help to multiplex 2 sets of RS232 to 1 RS232? When u mention I/O lines, do you refer to RS232. The 2 processors will be on - not disabled. What do you mean by allow voltage on its I/O line? I will be caution about the floating input pointer. Will either be using a PU or PD resistor to do that.

    My device only has 1 RS232 interface. Hence I would need to use this 1 RS232 interface to connect to 2 processors. I read that RS232 is a point-to-point topology (1 driver, 1 receiver). Hence I thought that 1 RS232 interface cannot be connected to 2 processors. I am not very sure about the reason for why RS232 can only be used for 1 driver 1 receiver. Is it because of fanout issue (the voltage will drop significantly if 1 RS232 is used to drive 2 sets of line? Or is it that if 1 processor send something out from its RS232, the other processor will also receive the signal on the same line, which is a output line. Hence receiving some signal on an output line may spoil the processor RS232.)

    Besides wanting the device to be connected to 2 processors, I would also like to have 2 such devices. In the end, there are 4 sets of connection.

    Device A (RS-232 Interface A)-> Processor A (RS-232 Interface A)

    Device A (RS-232 Interface A)-> Processor B (RS-232 Interface B)

    Device B (RS-232 Interface A)-> Processor A (RS-232 Interface A)

    Device B (RS-232 Interface A)-> Processor B (RS-232 Interface B)

    I should be trying out the following connection.

    1. Using splicing to connect Device A to the 2 processors. Do the same for Device B also.

    2. Using an external circuitry (buffer, multiplexer) to connect the Device to processor (so that only 1 processor is accessing the device at any one time).

  • 0 in reply to Wenjun Huang
    TI__Guru**** 165236 points

    Wenjun,

    The RS232 interface chips convert RS232 (>+/-5V) to logic level signals. We have many options.
    http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=543&uiTemplateId=NODE_STRY_PGE_T

    The RS232 lines are used in point to point communication, not  shared.
    Logic lines can be shared if only one output is active at a time.

     

  • 0 in reply to Ron Michallick
    Mastermind 7395 points

    Did you consider using RS485, which includes multidrop capability?

    Another possibility would be to use an MSP430 as RS232 hub.

  • 0 in reply to Ron Michallick
    Expert 1770 points

    I am actually using hardware that people have created to integrate into a system. The hardware is not built specially for this system but can be used. As RS485 is being used for other integration of card and the device is using RS232, I cannot be using RS485, must use RS232.

    The processor card I am using is actually using a RS232 transceiver (which is the link u have recommended). I would like to clarify the following.

    processor (uart interface) -> RS232 transceiver -> connecting cable -> RS232 device

    On the connecting cable, the signal is RS232. On the interface between processor and RS232 transceiver is logic logic level signal. Is that correct?

    If that is correct, I am trying to connect "processor (uart interface) -> RS232 transceiver" to the "RS232 device". Hence if RS232 is point to point communication, I cannot use your suggested method.

    Why is RS232 point to point? Could I violate this and try to use splicing cable to connect it to 1 driver, 2 receiver. What would be the repercussion?

    I am also intending to create circuitry (consisting of multiplexer and buffer) to connect it physically to 1 driver, 2 receivers. But the circuitry will make it logically connected to 1 driver, 1 receiver at any one time. Do you think it is feasible?

    Thanks.

  • 0 in reply to Wenjun Huang
    Mastermind 7395 points

    If you connect one driver to two receivers, you have:

    1) One TX from the PC. It could be spliced to two receivers.

    2) Two Rx from two devices. How can you possibly connect two drivers (one from each device) to a single Rx on the PC? The drivers are drivers, if you cannot shut them down, one of them could be trying to drive -5V, and the other one, +5V. Collision, data corrupted and danger of the drivers getting damaged.

    Regarding multiplexing, you would have to find a multiplexer that handles positive and negative signals.

  • 0 in reply to Avi Chami1
    Expert 1770 points

    Actually, I got 2 same processor cards, 2 same devices.

    The processor card has 2 RS232 interfaces, each to be connected to the 2 devices. 

    The device only has 1 RS232. I want to connect this 1 RS232 to the 2 processor cards. This will have the problem of potential spoiling the interface if both RS232 device try to send different signals to the processor card.

    So u r saying that the splicing method will not work. The circuitry method may work if I am able to find a multiplexer that handles positive and negative signals. Do you have any multiplexer to recommend? I believe an analog multiplexer which accepts voltages from -3 to -15V and 3 to 15V should work for RS232.

    What makes RS232 different from RS422 or RS485? Why is RS232 point-to-point, RS422 multi-drop (1 driver, many receiver), RS485 multi-point ( many drivers, many receivers, but only 1 driver can transmit at any one time)

  • 0 in reply to Wenjun Huang
    Mastermind 7395 points

    Specifically, RS485 and RS422 are designed and tested with multidrop applications in mind. They are designed and tested to withstand and survive a collision with no damage. RS-232 on the other hand, since it was never defined for that, are not designed nor tested (as far as I know) with collision requirements in mind.

    I don't know of a suitable MUX, in your case, and if I were going to do special purpose H/W, I would consider a microcontroller or CPLD. It will be not so more expensive to build, and would save a lot of pain while debugging (so I think).

    About RS485 and 422:

    http://www.ti.com/lit/an/slla070d/slla070d.pdf

     

     

  • 0
    Intellectual 350 points

    Avi Shalom,

    RS2323 its not acting as a bus. For multi  nodes you need RS485 instead RS232

    Thanks

    Haim