5
\$\begingroup\$

I currently have a very fragile solution to connect my embedded device to a PC's serial port. This is mounted on the PCB: http://www.digikey.com/product-detail/en/DF13A-6P-1.25H(50)/H3384-ND/530681. Its companion must be painstakingly attached to the wires of an existing serial cable, and placing any sort of strain-relieving product takes too much expertise. The wires often break off. Only five pins of the six are used. I may be able to respin the board this connector must be on.

I'd like to replace it with a Micro-USB B receptacle so that I can cut open easily-obtainable Micro USB cables and mate them to a DB9 connector. Is the 5th pin actually wired on most cables? I don't know much about the USB standard. Does this seem like a decent idea?

EDIT: This port is factory accessible only.

\$\endgroup\$
  • 1
    \$\begingroup\$ USB is 4 wires. If you can respin the board, you could add an FTDI chip and have it emulate a USB serial port right there on the board? I'm guessing there isn't much space though. \$\endgroup\$ – pjc50 Nov 5 '12 at 17:58
  • \$\begingroup\$ What is the 5th pin connected to, then? I don't think CTS and RTS are required, so I could get by with 4 pins. \$\endgroup\$ – Cat Zimmermann Nov 5 '12 at 18:20
  • \$\begingroup\$ Why not a more standard DB-9 or 25? Sooner or later someone will plug the device in a USB port. \$\endgroup\$ – jippie Nov 5 '12 at 19:06
  • \$\begingroup\$ There is no room for anything larger than the connector in the URL I posted. \$\endgroup\$ – Cat Zimmermann Nov 5 '12 at 19:27
  • 1
    \$\begingroup\$ If your main problem is the wires breaking off your serial cable, some heat shrink tubing or sugru around the connector will likely solve that problem and doesn't take much effort. \$\endgroup\$ – Grant Nov 5 '12 at 20:15
5
\$\begingroup\$

The ominous Pin #5 is usually the ID Pin and does not travel through the cable. If I remember correctly, this pin is usually shorted to ground, indicating a device device or open, indicating a host device. In other words: This pin decides if the device which is connected plays the host or device role.

If you use the other 4 pins, make sure that no user will really use a "normal" USB cable (and they will) as your device will most likely get 5V on the power pin (which is probably not what you want).

PJCs solution is in my opinion a good one (but requires you to change the board)

\$\endgroup\$
  • \$\begingroup\$ Thank you for the clarification. This is for factory use only, thankfully. \$\endgroup\$ – Cat Zimmermann Nov 5 '12 at 18:50
3
\$\begingroup\$

My recommendation is to find a suitable connector that is NOT a USB connector, unless you want to build a serial-to-usb adapter. The reason for this is one of your failure modes: If you put a USB connector on the thing, somebody will some day plug it into a USB port. When somebody does, they will wonder why it doesn't work, and if the device or their motherboard sees any damage, they will be pretty ticked off-- justifiably.

If you still feel compelled to do this, try to make sure that both the device and motherboard won't be hurt when somebody plugs it in to a USB port.

\$\endgroup\$
  • \$\begingroup\$ This is factory accessible only. \$\endgroup\$ – Cat Zimmermann Nov 5 '12 at 19:26
1
\$\begingroup\$

For factory debug/program, I like using FFC (flat-flexible cable): http://uk.farnell.com/jsp/displayProduct.jsp?sku=1012182&CMP=e-2072-00001000&gross_price=true (many many variants, that's just an example)

It takes up a fairly small area on the board. You'll want to buy or build a small breakout PCB on the other end of the cable - you can't solder to FFC.

My other solution which Tom L liked is "add an FTDI chip and have it emulate a USB serial port right there on the board": http://www.ftdichip.com/Products/ICs/FT230X.html ; this is very user-friendly but requires adding an IC.

\$\endgroup\$
1
\$\begingroup\$

If you do use a USB connector, it would be wise to make sure that the attached circuit is designed such that no damage will occur if it is connected to a regular USB port. Eventually someone will try it. Ideally though, I'd recommend using a different connector altogether.

Consider using an RJ11 or RJ45 connector. They're not much larger, and they are very commonly used for serial connections, so it will be recognizable, and off-the-shelf cables are available from many sources. Many Cisco routers use this approach for their console connections.

Here's one example of an affordable RJ45-DB9 adapter: https://www.amazon.com/dp/B06XFYYXDF/

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.