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Invert the PICAXE AXE027 FTDI cable Serial Data going into a chip?

Other Parts Discussed in Thread: SN74LVC04A, SN74HC04

I have a PICAXE AXE027 FTDI cable and the Serial Data at 9600 BAUD I'm sending through the cable I think needs to be inverted to be compatible with the SpeakJet chip that needs to receive the data.  

I am not an electronics engineer so I'm trying to find the simplest solution to invert 9600 BAUD serial data as it flows from the cable to the chip.  

The SpeakJet chip can run on 3.3V to 5V no problem.  I normally tie the SpeakJet chip to a 3-AA cell battery pack connecting grounds together.  

I'm looking for a simple schematic with a part number and if there's a resistor, the value of the resistor.  


  • Hello,

    Short answer: You need a logic inverter such as SN74LVC04A -- assuming your signal is 0V for a low and 3.3 or 5V for a high.

    Long answer: I will need your help in getting some info for the signal you want to invert to be certain of what you need.

    You mentioned 'serial data' going over an 'FTDI cable' at 9600 baud -- this brings up a whole world of questions for me. I will try to keep them simple and specific.

    Is your data signal single-ended or differential? (single-ended means all signal lines share a ground, differential means there's a pair of lines for each signal) For example, USB has a D+ and D- line, which means it has a differential signal.

    What voltage is your signal operating at -- you mentioned 3.3V and 5V for the chip operating voltage, does that mean your communication channel is operating at 0V for a low and 3.3V or 5V for a high?

    I would also want to know the edge rates of the signals when they reach the device that will be doing the inversion. Do you have access to an oscilloscope to give us images of the signal?
  • Hi,

    All I know is that the output is serial data normally to a PICAXE 28X2 module or PICAXE chip (which are PIC chips)

    Using a SparkFun FTDI cable (www.sparkfun.com/.../9718) I can send 9600 BAUD serial data from my PC to the SpeakJet chip no problem. I'm guessing the AXE027 cable (www.picaxe.com/.../axe027.pdf) is just inverted bits. So I want to bread board a very simple circuit to test this.

    Here's a short video showing the signals from the PICAXE AXE027 cable;
  • Looking at the schematic for the FTDI cable you mentioned, there's a chip in there called the FT232R, which converts USB D+/D- lines into UART single ended 5V signals.

    If you want to invert one of those lines, the simplest device to achieve that (assuming you want something in a PDIP package) is the SN74HC04.  This is a standard CMOS inverter that will operate at 5V.  This particular device has 6 inverters in it, so be sure to properly terminate any unused inputs (just ground unused inputs, leave unused outputs disconnected).

  • Hi, 

    I only need to invert the output as I'm not using the input.  Isn't there a simple transistor or single bit inverter chip that can do that?