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Graynomad
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« on: February 11, 2011, 12:52:37 PM »

I've struggled for months with the decision about what cable/connector combinaiton to use.

Initially I wanted something that didn't require special tools and could use any old wire and I still like that in principle. The good thing about this is that any wire can be used and large wire can carry more current, therefore handle more Nodes.

However screw terminals are large and ugly and there's no neat way to attach wires or daisy chain Nodes. You can get plug-in versions but you still have the ugly wire connections that are prone to falling out.

I feel a modular connector should be used, there's the RJ45/Cat5 combination which I like but the wire is large, as are the sockets. OTOH it has 8 conductors and so can provide quite a lot of power.

My current preferrence is RJ11 and flat telephone cable, either 4 or 6 way. Using 4-way jacks you can fit two of them (for easy daisy-chaining) and a screw terminal block (for the IO connection) onto a 1x1" Node.

I know the modular approach requires a crimper to make up cables, but these are common and cheap and the resultant connection is very reliable. Also the flat cable can be easily run just about anywhere, including through the seal in a fridge door.
« Last Edit: February 11, 2011, 01:24:32 PM by Graynomad » Logged

Rob, aka the GRAYnomad
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« Reply #1 on: March 08, 2011, 01:44:09 PM »

Hey Rob,

RJ11 is my preference for this too.

I like them 'cuz their reasonably cheap, you can find them everywhere
and the have the snap lock.

Down side is that someone may accidentally plug
busnet into a phone jack. Oh well. :-)

Another interesting thing: you can plug a RJ11 into a RJ45.
So if you need 2 extra optional wires then you could have
RJ11 and RJ45 mix and match, just make sure the 2 outside wires
are the optional ones.

One word of caution: be careful about the tabs.  (you can buy
the socket either tab up or tab down)  I have used equipment
where they have put the tab on the wrong side and it was
impossible to get my fat fingers in there to press the tab.
Pain in the rear!

cheers, John
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Graynomad
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« Reply #2 on: March 08, 2011, 02:09:26 PM »

Hi John and welcome.

Interesting about RJ45, I have figured that RJ11 6P6C could be used to up the power conductors to 4, moving to RJ45 would be even better of course.

Another thing about tabs, each Node has two jacks to allow easy daisy chaining, however if you connect the two jacks in parallel on the Node you have to mount the connectors on opposite sides of the cable, therefore a standard telephone wire won't work.

The alternative is to use normal cables and cross the signals over on a Node PCB.  I don't like this option though because the two jacks are not the same, you would have to have an "upstream" and "downstream" jack and this is prone to error.

Any thoughts?
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« Reply #3 on: March 09, 2011, 01:18:16 PM »

Any thoughts?

Hi Rob,

Interesting problem!

So, just so I understand, the problem is where the
pins get re-mapped
   1 -> 4
   2 -> 3
   3 -> 2
   4 -> 1
(i.e. the usual straight v.s. cross-over issue;
more of an issue with flat cables)

Here's an idea... use the following pinout:
   1 -> RS485 B (+)
   2 -> V+
   3 -> GND
   4 -> RS485 A (-)

then put a full wave rectifier on each board
(cheap, since we're dealing with small currents and SMT)

So if the V+ and GND get switched, no problem,
the bridge diodes will take care of it.
(one downside: the voltage drop)

Now the trickier part is if A and B get switched on the RS485 link.
Essentially this means mark/space gets reversed.

One way to handle this is to do it in software.
When the board boots up, have it "call home",
i.e. contact the master.  If it doesn't work,
then try inverting the bits on the message
and see if that works.  Then just keep in memory
a flag telling whether to flip the bits or not,
based on this test. (obviously some details would
have to be worked out).

Or another way is to monitor the bus, and somehow
deduce if A and B are inverted (maybe looking at
idle time).

It's really hard for me to know if this will
work since I haven't read up on the details
of your protocol.

Anyways... just throwing this idea out there.

.........

On a related topic, you are thinking of using a 4P4C
connector, due to size.  One downside of 4P connectors
is here (in Canada), 6P cables are much more common,
since their just regular phone cables.  4P cables are
typically only used between the handset and the phone.

I did some checking over at molex.com and found
the following dimensions:
   4P4C single   width .440" depth .810"
   6P6C single   width .520" depth .810"
   6P6C double   width .985" depth .810"

I'm not sure if you want your board to be 1"x1"
or if the enclosure should be that size.
Anyways, maybe a 6P6C double (2 port) might work,
since it's width is .985"

The advantage of going with 6P6C, is that you
could use the extra 2 lines for more power.
Maybe one pair at 5V, the other at 12V.
Perhaps this could be used for stuff like LED lighting
or for running small fans, servos, etc..
Might give you an extra 5 W (12 V @ .5 amp).

And you could even push this further, by supporting
8P8C connectors by using the last pair for 48 V.

In fact you could support all:
  4P4C : 5v
  6P6C : 5, 12 v
  8P8C : 5, 12, 48 v
and have them all work with the same jack
on the master, since the smaller connectors fit
into the larger jacks.

OK, maybe I'm getting carried away.  Grin

Anyways... hope this helps in some way.
I don't mind doing stuff like this,
if you think it's helpful to you.

cheers, John
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« Reply #4 on: March 09, 2011, 07:38:29 PM »

Quote
(i.e. the usual straight v.s. cross-over issue; more of an issue with flat cables)
Correct.

I like the idea of allowing all plug/jacks from 4/4 to 8/8, this means that the essential signals have to be in the centre and others "grow outwards".

This does not preclude your idea of the rectifiers and auto signal polarity but that does add some software complexity and (relatively speaking) a lot of hardware. I want my original Nodes to be 1x1" as you say, and that

a) doesn't allow for any larger than 4p4c jacks
b) already needs about half the PCB for the core logic, leaving a very small area for the application.

So far all applications I have on the drawing board fit but adding a rectifier or 4 diodes may be a problem.

On larger Nodes of course this wouldn't be a problem.

When a Node boots up it has to sync with the network. It does this by waiting for a space (idle) of 13 bit times or more. So this could be tried for a second or two and if no joy wait for a mark instead. So I don;t see that being too difficult.

The 485 transceivers I plan to use on my Nodes are auto direction chips (they don't need a TX enable control), I'd have to have a look and see if they would allow reversing the line polarity. I don't know much about 485 at this low level.

I'll have a think about this because forcing cables to have connectors on each end opposite to each other means you can't just rock into Tandy and buy a phone cable.

As I said I really like the idea of being able to use anything from 4/4 to 8/8 and standard cables, maybe it's worth the extra complexity to make the final product simpler.

Quote
 6P6C double   width .985" depth .810"
Can you point me to an SMD version of this? I couldn't find to fit < 1" except 4/4. Also I need a bit of room at the end for screw terminal blocks. 0.19" might be enough I'll have a look.

Quote
The advantage of going with 6P6C, is that you could use the extra 2 lines for more power.
Maybe one pair at 5V, the other at 12V.
The spec calls for local regulating of a VBAT up to 18v. I do plan to use VBAT on Nodes to power relays, lights etc as you say.

The only issue with this is that the local regulator has to handle the heat dissipation, but that can be dealt with by the Node designer as he knows the current draw of the Node.

The only reason for 18v is that gave some headroom over a normal vehicle "12v" and some LIN chips (I originally used LIN transceivers) aren't designed for any higher.

I think the 18v is a reasonable limit though, and will probably spec a nominal 12v with 18v max. This allows for a lot of voltage drop over the network.

This of course means that a drop over a diode bridge won't be a problem.

So the four wires are

GND
D+
D-
VBAT

Plus of course more GNDs and VBATs if larger connectors are used.

OK I'm going to have a think about this polarity independence, and go looking for the world's smallest SMD rectifier.

Keep thinking, it we can get this to work it will be a big improvement.

« Last Edit: March 09, 2011, 08:22:07 PM by Graynomad » Logged

Rob, aka the GRAYnomad
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« Reply #5 on: March 09, 2011, 08:27:04 PM »

Quote
go looking for the world's smallest SMD rectifier.
I found them.

Avago HSMS-280x series in a SOT-143 package. I actually already had a single diode in series with the power as a protection method, this bridge is probably not much bigger and gives us full polarity independence for the power.

Or does it?

What if you plug crossed and non-crossed cables into the same Node. The Node is OK but do you short the power supply?
« Last Edit: March 09, 2011, 08:30:17 PM by Graynomad » Logged

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« Reply #6 on: March 10, 2011, 03:56:33 PM »

Quote
What if you plug crossed and non-crossed cables into the same Node. The Node is OK but do you short the power supply?
As long as there is only one power supply it doesn't matter, the VBAT and GND will swap pins after every Node but that doesn't matter.

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« Reply #7 on: March 11, 2011, 01:52:39 PM »

Hi Rob,

I've been thinking more the connectors.

If you want your nodes to be 1"x1", it is pretty difficult
to use the modular connectors.  Basically they're too big!

Yes, 4p4c might work, but the biggest downside is that
you have to make your own cables since it's hard to
find cables with 4p4c connectors.  So making a lot of
cables for daisy chaining will be a pain.
(with 6p6c there is no problem finding cables and they're cheap.
E.g. I can get a 0.5 meter cable for $0.36)

========

Here's a new idea:

Have you checked out the miniture connectors from Phoenix:
  http://www.phoenixcontact.com/global/products/54143_54148.htm
(there's a movie at the top of that page that gives a summary of their products)

Their PTPM connector looks interesting (near the bottom of the above page).
No need to buy a crimper! SMT technology.  Small - a 4 pin connector is 7.5 mm wide.

========

Another idea is to use a small screw terminal block.

Then if people want to have pluggable cables, they could mount
it inside a surface mount box using the screw terminal block:
   http://www.shophyperline.com/item_content.php?p_=127
(cheap, $0.86)

========

OK, there are some more ideas for you to chew on. Smiley

cheers, John
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« Reply #8 on: March 12, 2011, 12:25:47 PM »

Thanks again John.

It's probably time to make an executive decision eh?

Those Phoenix connectors look good and I like the idea of a "no tools" connection, but I hate the look of discrete wires and if a multi-core wire is used I see no way of having a neat termination without using heat shrink or something (and that's not particularly neat).

This is actually the reason I revisited this connector issue as my original connectors were the Phoenix plug in terminal blocks, easy in that all you need is a screw driver, but ugly.

Quote
Another idea is to use a small screw terminal block
This is exactly the reason I think RJxx is the way to go, there is no end of wall plates, junction boxes etc available at any corner shop (well almost).

For example that box you linked to could make quite a nice housing for a Node.

So I think the way to go is as follows

  • The default connector is RJ11 6/6.
  • The wiring is "centre out", therefore 4/4 can be used for tiny Nodes and 8/8 can also be used for more current-carrying ability.
  • Nodes will have diode bridges for the power and be able to auto detect/correct the signal polarity, this allows the use of crossover or straight cables with no thought about what plugs into what.

NOTE: Small diode bridges can drop up to a couple of volts, this will affect the spec'd VBAT level but that is a small price to pay I think.
 
Have we finally got there?

« Last Edit: March 12, 2011, 12:28:47 PM by Graynomad » Logged

Rob, aka the GRAYnomad
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