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I have a USB to TTL Adapter. The Adapter uses the CP2102. I noticed that the CP2102 seems to derive it's power from the USB V+. My problem is that I need to supply power to the USB. The reason is that I need to drive a NULL USB cable. The transceivers (FT232R) requires 5V. I connected both the 3.3 and 5V and measured the Voltage at the USB and it is 3.3V. My plan is to just jumper the 5V to the USB V+. I don't have a schematic for the USB-TTL adapter. I believe This should work.

Adapter: https://www.amazon.com/HiLetgo-CP2102-Converter-Adapter-Downloader/dp/B00LODGRV8

NULL USB: https://www.amazon.com/FTDI-USB-NMC-2-5M-CABLE-ASSEMBLY-MODEM/dp/B00HKJSSQ2

FT232R Datasheet: https://www.ftdichip.com/Support/Documents/DataSheets/ICs/DS_FT232R.pdf

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  • \$\begingroup\$ What are you trying to do? Connecting that USB cable to CP2102 makes no sense, as they are both USB devices so they both need to be connected to a USB host to work. You can't connect two USB devices or two USB hosts together and expect it to work. \$\endgroup\$
    – Justme
    Commented Feb 9, 2021 at 17:56
  • \$\begingroup\$ just use a USB extension cable instead of the null USB cable ... the result would be the same \$\endgroup\$
    – jsotola
    Commented Feb 9, 2021 at 18:14
  • \$\begingroup\$ justme: One Device has USB to Serial Already. 2nd Device has TTL to USB. Between the two is a "NULL" USB cable. Why does one have to be a host? Connecting 2 computers together would work. Its the same concept, only the computers in this case are serial devices. \$\endgroup\$
    – Christopher J. Holland
    Commented Feb 9, 2021 at 20:06
  • \$\begingroup\$ jsotola: I believe you would need a router as you cant just connect USB Tx to USB Tx and USB Rx to USB Rx. That is why I need to use a NULL USB cable. \$\endgroup\$
    – Christopher J. Holland
    Commented Feb 9, 2021 at 20:07

1 Answer 1

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The USB protocol makes a strict distinction between hosts and devices, and does not allow two of the same kind to communicate with each other. A USB bus always consists of a single hub, and an arbitrary number of devices that talk only to the host.

That null modem cable contains two devices, i.e., two USB-to-serial converters, where the serial data lines are connected directly to the other device. This cable works only with two hosts.

If you want to connect a microcontroller (or whatever it is that uses TTL) to a computer, use the USB-to-TTL adapter together with a plain USB-A-to-USB-B extension cable.

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  • \$\begingroup\$ I have 2 computers that communicate serially using a null USB cable. So, that is Host-to-Host communications. I am trying to get to serial devices to communicate to each other over usb null cable. I think, not totally sure, but isn't the USB cable just differential? There is no smarts in a cable. I don't get it. \$\endgroup\$
    – Christopher J. Holland
    Commented Feb 12, 2021 at 13:09
  • \$\begingroup\$ As I said, that cable contains two device chips. \$\endgroup\$
    – CL.
    Commented Feb 12, 2021 at 14:50
  • \$\begingroup\$ One side is using a USB83340, which is a transceiver. It has a pin grounded, which means that it is setup to be a host. The CP2102 on the other side is a transceiver as well, but I guess it only acts as a client. The USB83340 is not being controlled in any way and neither is the CP2102. Perhaps the Host is supposed to control communications, but the client initiates communication. I'm not sure how to make this work of if it ever will. It just kind of blows that before, with an RS232 driver, I could just swap RX with TX and be done with it. \$\endgroup\$
    – Christopher J. Holland
    Commented Feb 12, 2021 at 17:13
  • \$\begingroup\$ The USB83340 is a ULPI PHY; it requires a USB controller with a ULPI interface to work. There must be another chip. \$\endgroup\$
    – CL.
    Commented Feb 12, 2021 at 20:07
  • \$\begingroup\$ CL, the USB83340 is connected to a chip. The operations happen in the background / hardware, like JTAG. (There is no source code). So that is supposed to act like a host and the 'CP2102' was supposed to act like a slave, but it did not work, so I gave up on that. I eventually got it working another way. So, it only took 2 weeks to get to USB devices to communicate with each other. [p.s. I think connecting 2 hosts is possible with OTG. From what I read, OTG can act like a host or client. ( I am a bit confused on who decides which will be the client and who will be the host?) ] \$\endgroup\$
    – Christopher J. Holland
    Commented Feb 16, 2021 at 19:15

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