4
\$\begingroup\$

Take, for example, this DB-25 connector from Digikey.

http://search.digikey.com/us/en/products/5747846-4/A32126-ND/808681

Look at the features:

features | Board lock, ground strap

I understand what "Board Lock" is - the pins will puff out once inserted into the PCB so that it is held tightly in place.

The other feature is "Ground Strap". I've tried to do some googling but I get a lot of noise regarding wrist straps. What is it, exactly, and what is its purpose? Why would I want/not want that feature for my connector?

\$\endgroup\$
2
  • 3
    \$\begingroup\$ You've got board lock wrong. Board lock means that there are mechanical parts that hold the part to the board by biting into it. These are usually mechanical parts, not the signal pins; in your example, they're the two large four-leaf things to either side of the signal pins. What you're thinking of, where the signal pins are puffed out so each one looks like a needle with an eyelet, allowing the connector to be installed on the board without solder, is called press-fit. It's used in VME backplanes and similar applications where the board is so thick that wave solder would not work. \$\endgroup\$
    – Mike DeSimone
    Commented Dec 1, 2011 at 4:05
  • \$\begingroup\$ My bad, I was ambiguous when I said "pins". I was actually talking about the non-signal pins connected to the outer shell that are used for mounting on the board. \$\endgroup\$
    – ajs410
    Commented Dec 1, 2011 at 21:46

2 Answers 2

Reset to default
8
\$\begingroup\$

I've never heard it called "ground strap", but if they mean the outer metal shell and its connectibility via the two end pins to connect to a chassis ground, that's very useful.

Why? Let me tell you a story....

Once upon a time, there was a computer called the Commodore 64. It had two plastic DB9 ports for joysticks.

We had one of these when I was young, and it brought many hours of joy, until one crisp winter day, when I went to plug in a joystick into the DB9 connector and zap, a spark jumped between plastic pieces to one of the connector pins, and that joystick port no longer worked.

My father and I opened up the case and looked at the Commodore circuit diagram in the back of the computer's reference manual, and he figured out which chip must have been damaged. We managed to replace it, and off we went. Joy was restored to the household.

Then a few years later the same thing happened -- zap -- and it must have damaged more than the input chip for the joystick port, because the computer no longer worked again.

We bought a Commodore 128 to replace it, and made sure to be extremely careful when plugging/unplugging the joystick connectors.

The moral of this story, is if you wish to protect your DB9/DB25/whatever signals from electrostatic discharge (ESD) events, use a connector that has a metal shell, and tie the metal shell to earth ground via the device's power cord, so that ESD events are likely to surge to the shell rather than to the pins. (If you have no power cord connection, at least the ESD event will surge to the device's chassis.)

\$\endgroup\$
3
  • 5
    \$\begingroup\$ It's a little more complicated than that. You not only need to connect the shell to the earth ground, you also need to connect earth ground to signal ground through a high-value resistor, such a 1 M to 10 M, to prevent the two grounds from simply becoming a capacitor. Sometimes a capacitor is added to provide a low-impedance path from signal ground to shield in a shielded-cable application to reduce EMI. \$\endgroup\$
    – Mike DeSimone
    Commented Dec 1, 2011 at 4:08
  • 1
    \$\begingroup\$ @MikeDeSimone that is actually exactly what is done for the USB connector. 1M connecting shield to signal GND, in parallel with 4.7 nF cap. I still can't get over the fact that they mass produce connectors which float the shield... \$\endgroup\$
    – ajs410
    Commented Dec 1, 2011 at 21:54
  • 1
    \$\begingroup\$ You mass produce connectors which float the shield because it is a lot more expensive to put those components into the connector than onto the board which mounts the connector. Same reason that not all Ethernet connectors have built-in isolation magnetics. \$\endgroup\$
    – Mike DeSimone
    Commented Dec 1, 2011 at 22:40
3
\$\begingroup\$

It's what connects the outer portion of the connector to ground. This outer portion is usually connected to the shield on the cable. It maintains continuity of the shield right through the connector.

\$\endgroup\$
3
  • \$\begingroup\$ I thought that's what the ground strap meant, but I began doubting myself because grounding the shield is such a common-sense thing to do that I'm actually astonished that connectors are mass-produced which float the shield. \$\endgroup\$
    – ajs410
    Commented Dec 1, 2011 at 21:49
  • \$\begingroup\$ You get DB connectors that are all plastic - no shield at all. Mainly in older equipment (like the C64 mentioned in another answer). \$\endgroup\$
    – Majenko
    Commented Dec 1, 2011 at 22:15
  • 3
    \$\begingroup\$ Many people get shield grounding wrong. You are supposed to connect shield to signal ground at one point, in the I/O area of the board. If you connect it at multiple points, you might send some of your return currents through the shield instead of signal ground, creating EMI. \$\endgroup\$
    – Mike DeSimone
    Commented Dec 1, 2011 at 22:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

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