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I have question about generic USB TTL Serial cables, FTDI FT232R-based. The cables differ in voltage, 5V, 3.3V and output current.

USB TTL Serial Cables

I need a cable for a USB to serial converter, for a modem with 3.3V voltage. Only 3 wires are required: Tx, Rx, GND (VCC not used). There are two models:

  • TTL-232RG-VSW3V3-WE: +3.3V / 50mA output.
  • TTL-232RG-VREG3V3-WE: +3.3V / 250mA output.

In my particular case the power output rate doesn't matter, as VCC is not used. But I may need this cable in the future for other electronic devices, like modems and routers, which may need VCC. Which from these two models is better for use with CPE devices (routers, modems, set-top boxes), the one rated at 50mA or 250mA?

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closed as off-topic by Joe Hass, Chetan Bhargava, Matt Young, placeholder, Kaz May 27 '14 at 17:36

This question appears to be off-topic. The users who voted to close gave these specific reasons:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – placeholder, Kaz
  • "Questions seeking recommendations for specific products or places to purchase them are off-topic as they are rarely useful to others and quickly obsolete. Instead, describe your situation and the specific problem you're trying to solve." – Joe Hass, Chetan Bhargava, Matt Young
If this question can be reworded to fit the rules in the help center, please edit the question.

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The difference is that some cables have a built in LDO (linear regulator) supplied from the USB power for 3.3V that can support up to 250mA, whereas the others use the built in LDO on the FT232R. Quoting the FT232R datasheet:

+3.3V output from integrated LDO regulator. This pin should be decoupled to ground using a 100nF capacitor. The main use of this pin is to provide the internal +3.3V supply to the USB transceiver cell and the internal 1.5kΩ pull up resistor on USBDP. Up to 50mA can be drawn from this pin to power external logic if required. This pin can also be used to supply the VCCIO pin.

In general, if you are using it to flash the firmware of some set-top box, it's unlikely you will need power from the USB anyway, since all the components on the set-top box should be powered by the power supply of the box (be sure, of course, to connect the ground through the RS-232 interface). In that case, I suspect your answer is just whichever cable is cheaper.

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