Flex USB Bit cable: Raspberry Pi GIPO Input?

Alan · March 2020

Has anyone used a Flex USB Cable BIT cable directly connected to a Raspberry Pi GPIO data input pin?

If yes, do you use pull-up or pull-down resistors? Any details on making the connections (VCC, GND, GPIO) pins from the Flex to the Rasperry?

Alan
WA9WUD

Neal Pollack, N6YFM · March 2020

FTDI USB to Serial cables can be ordered for many different logic voltage levels.
You must be careful to purchase the proper voltage cable for your logic, else risk
blowing out the chips. Some FTDI cables put out 5 volt serial data, some 3.3volts,
some for FPGA chips can come in 2.5V, 1.8V, and 1.0V logic levels.

FTDI makes a model that is specifically for use with the Raspberry Pi 3.3volt logic,
and has the 2.54mm female push pin sockets that will slide over the Raspberry Pin
GPIO header pins;

Model:  TTL-232R-RPi
Data Sheet:  https://www.ftdichip.com/Support/Documents/DataSheets/Cables/DS_TTL-232R_RPi.pdf

Example Price:  https://www.ebay.com/i/303183315509

If you want the model that brings out more of the misc I/O lines for the COM port,
take a look at this selection list;
https://www.ftdichip.com/Products/Cables/USBTTLSerial.htm

Regarding pull-up or pull down resistors, that has to do with how you design
your circuit and programming. For example, with a GPIO input used for a push-button
switch, most of the examples you google for will use a pull-up resistor. When you push
the switch, it is wired to short the GPIO to ground. Your software would look for a change
from a 1 to a zero (ground). There are other types of circuits that may need a pull-down.
It always has to do with what the normal/idle voltage level is, or needs to be. There is
no simple one-size-fits-all answer. You use a pull-up or pull-down only if your design
requires it.

This may or may not help;
https://learn.sparkfun.com/tutorials/terminal-basics/all

Best WIshes.

Neal

Alan · March 2020

Thanks, Neal.

In my case, I am using Node-Red and the GPIO pin will be a logic true or false (3.3v or 0V). I have used this same configuration with a relay contact, 3.3v, and a pull-down resistor, so I am familiar with the concept.

I searched the Flex documentation for information on the BIT cables. Found how to set up the cable and which color corresponds to each BIT, but nothing on the voltage levels of the outputs, high and low. I have used the Flex BIT cable to directly operate a 5V relay board.

Is the Flex 3.3V high/ 0 V low? Or, does the Flex voltage, hi/low not matter because the chosen FTDI cable specification type determines the high/low voltage to the Rasberry Pi? Does an FTDI cable provide DC isolation between the Flex and the input to the GPIO pin? I do not want to provide any "sneak" paths that could cause the Flex side of the USB cable to be damaged. Does adding a powered USB hub provide the necessary DC (VCC) isolation? My major concern is to not damage my Flex 6600 with some sort of DC backfeed from the Raspberry Pi.

Node-Red can adjust for either pull-up or pull-down. I just wanted to ask the community if there were any unusual differences when using the Flex BIT cable instead of a set of relay contacts connected to the Raspberry Pi GPIO logic pin.

It looks to me, that I need a 3.3V FTDI cable for my application (TTL-232RG-VREG3V3-WE).

Am I missing anything?

Alan
WA9WUD

Neal Pollack, N6YFM · March 2020

Yes, that is correct. Ignore the Flex USB port voltage, it is the FTDI adaptor cable that determines the logic voltage at the end of the wires you hook up to a circuit. The FTDI model you mention above will work fine. You can verify any other bit cable using a standard DMM/VOM to measure the voltage on the wires coming out of it (when you switch from logic zero to logic high, of course).

Cheers,

Neal

Flex USB Bit cable: Raspberry Pi GIPO Input?

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