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SN75155 RS-232 Driver Issue

John Nobile
Prodigy 40 points
Other Parts Discussed in Thread: SN75155, TRSF3221E

I am trying to use this as just a driver, but I cant see anything on my hyper terminal. The chip works fine as a receiver, since I can send text from my hyper terminal to the chip and see the output on an LCD screen. I have the chip running at 5VDC. Is there anything else I need to add to the circuit to get the chip to work as a driver?

  • 0
    TI__Guru**** 164796 points

    Hello John,

    The SN75155 requires dual supplies; the range is +/-4.5V to +/-15V.  Do you have a -5V on pin 1 (VCC-)?
    If you only have a +5V supply, I suggest a single supply devices , like the TRSF3221E .

    Regards,
    Ron Michallick

  • 0 in reply to Ron Michallick
    Prodigy 40 points

    Right now, I had it running at 5VDC on pin 1. Would I need to have a 15VDC powersupply hooked up to pin 1 in order for this thing to operate properly?

  • 0 in reply to John Nobile
    TI__Guru**** 164796 points

    John,

    There should be a negative supply on PIN1 (VCC-) and a postive supply on pin 8 (VCC+).

    Pin 1 = -5V and Pin 8= +5V is good.
    Pin 1 = -12V and Pin 8= +12V is better
    Pin 1 = +5V and Pin 8= +5V is not good.

    Regards,
    Ron Michallick

  • 0 in reply to Ron Michallick
    Prodigy 40 points

    Sorry, yes that is how its set up, -5 on pin 1, +5 on pin8.I have the signal from my microprocessor going to pin 2, and pin 7 going to the RS232 connection on my PC.No other parts hooked up to the circuit. Still doesnt seem to work, even at the 12VDC =/

  • 0 in reply to John Nobile
    TI__Guru**** 164796 points

    John,

    Now that the supply voltage have been verified, it is time to measure the voltages on the 'DA" and 'DY' pins.

    Measure the 'DA" input voltages. Verify the VIH is at least 2V and VIL is less than 0.8V.

    If the 'DY' voltages meet the VOH and VOL values on page 4 of the data sheet, check the RS232 wiring to the PC.
    If the voltages are bad, try unplugging the RS232 cable and measure again. 
    If still bad, try changing the SN75155.

    Ron

     

  • 0 in reply to Ron Michallick
    Prodigy 40 points

    I hooked up a scope to the DA and DY pins to make sure the voltages were correct. DA was between 0-5VDC. DY was a little high with the rs232 connected to it, but removed, it was within the acceptable range. I still see nothing on my hyper terminal. When I checked the DY signal, it looks like it is stuck high, at around 5VDC. The DA signal is sending the message, so the levels go between 0-5VDC. I went through 10 of the chips that I had, to make sure that the one wasnt bad, but they all showed the same values. My circuit configuration is: -5VDC to pin 1, +5VDC to pin 8, Serial in from Microprocessor to pin 2, serial out to pin 7, GND of the serial connected to pin 4.  Should I have the GND pin also put to -5VDC?

  • 0 in reply to John Nobile
    TI__Guru**** 164796 points

    John,

    When using the +/-5V supply, the DA signal should be less than +4.3V.
    If it is stuck at 5V, check for a board short between pins 7 & 8. (DA and VCC+)

    The device schematic shows a base emitter (diode) drop then 360 ohms (60 + 300) when driving high. So DY(high)=VCC+ - 0.7V - Iout*360 ohms.

    Ron

     

     

     

     

  • 0
    Prodigy 10 points

    I see that this is old and may have been worked out but my 2 cents? I think all you need to do is turn on the local echo in Hyperterminal. In Hyper Terminal go to File - Properties. In Properties choose the Settings Tab and then the  ASCII Setup button. In ASCII Setup check the box  "echo typed character locally".