Recommended connector power and ground pin locations

[TimB]

Hi all

I'm looking to advice on the best connection pinout for power and grnd

I have a 4 way connector JST HX style. Currently for the pin out I have

1 = 5v
2 = signal 1
3 = Signal 2
4 = Grnd

I'm looking to select a cable to wire them up and thought about Tel cable as its has the 4 wire. However it is
1 Grnd
2 Power
3 signal 1
4 Signal 2

It has me thinking now I should be using that config. Originally I was thinking keep the 2 power lines apart to prevent shorting due to a stray strand but now I'm thinking but it they try and stuff the connector in the wrong way..

What is the right wiring layout?

[Wimax]

Hi Tim,

I think that the first is better, it sounds as a USB 2.0 pinout.

Max

[JonW]

It depends if Sig1 & Sig2 are coherent (ex differential etc). If they are keep them together and I would opt for

VCC
Gnd
Sig1
Sig2

or

VCC
Sig1
Sig2
Gnd

Keeps any signalling away from VCC that could be fed back.

If they need to be isolated

VCC
Sig1
Gnd
Sig1

Decouple 5V pins with 10pF and 10nF to create a broadband short on the VCC in both cases. 

For reverse polarity, use a polarised connector or a diode and isolate the signalling.  If the cable length is fairly long or in an industrial environment I would consider galvanically isolating the signalling and would definitely add low capacitance ESD protection on VCC and signalling
You can also use a high-current ferrite bead to isolate the connector Gnd to the PCB Gnd.  This is pretty standard on grounded connector shells.  If you are concerned about shorts then you can look at using a current limiting device to feed VCC onto the cable.  If cost is not a major concern (ie very high volume) then I find it's best to go overboard on the protection as if it can go wrong, it will bloody well go wrong in some instances.  If current is low and cable length short, look at adding a current limiting resistor in series on the 5V line, this sometimes works better than a diode as short disables the circuit as VCC is forced to GND through the resistor loss and adds further protection.

This reminds me of times when designing that you see an anomaly or instability under certain conditions and you ignore it as a one-off.  Experience has taught me that these always come back and bite you on the ass in the future. 

[TimB]

Thanks JonW

The signals are i2c lines and pulled up by resistors
The devices on the other end run on 5v.
No line is >50cm

This question come about because the tel cable colours are grnd,5v,sig1,sig2 Also I remembered that fans I use are the same.

Tim

[JonW]

Some reading this may think ah its ok and won't happen.  I have developed hundreds of Satellite TV-based products built in millions like STB, Routers, LNB, and Transceivers and you will be amazed at what gets returned when someone has rammed something where it doesn't belong, this costs time and causes issues with the customer base.

Just try to protect it in some way vs your cost and spend some time trying to make it go wrong

 

[charliecoutas]

JonW: At our museum we have built some radio receiver emulators: the user turns what looks like a tuning knob and hears 2radio stations2. They are actually turning an encoder and an 18F26K22 chooses a suitable audio track. What we didn't realise was how strong some people are. The volume control pot is frequently destroyed by people cranking it way, way beyond what its designers ever expected.

"..spend some time trying to make it go wrong" is such a good suggestion!

Charlie