Design Help

[BOB_SANTANA]

Hi Everyone

I am writing to pick up some advice from you on a project that was presented to me.
I am working a design that would be fitted in trailers. The aim of the project is to have an LCD display in the cab of the lorry that would tell the driver the length & height of the trailer on the display.
But it is not as simple as if sounds.
My client does not want ANY wire connection between the cab and the trailer and does not want to use wireless.
The way he wants this to work is that there would be 2 units one in the cab and the other in the trailer.
The unit in the trailer would be programmed with the length & height etc at programme time and the data would be stored in the internal eeprom of the pic.
So all the trailers would have their data stored there so they can be used with any cab.
The data stored in the trailer would need to be sent to the unit in the cab
But there is only a single wire connection between the cab & trailer (Which is the reverse line when not in use)
Question How can i communicate between the pics using a single wire to sent the data ?
Is this possible or is he asking for too much ?
Personally i would go for wireless using RF modules but he don't want any cable to be touched apart from the line i talked about.
Is there a way of doing this ? Regards.

Bob_santana
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BOB_Santana

[ckielstra]

I don't have a clear picture of the project:
- With only a single wire, how is the device in the trailer going to be powered?
- This single wire that's is available, what other uses does it have? I mean, you say it 'is the reverse line when not in use' and I don't understand what you mean by that. What is a 'reverse line' and what do you mean by 'not in use'?

Maxim has a technique called 1-wire. It comes in two flavours, a PCB mounted chip or in a small rugged casing. http://www.maxim-ic.com/auto_info.cfm
These '1-wire' devices use one wire (a combined power/data line) and a ground connection. The tiny rugged devices come in several flavours, including a version with programmable memory. These sound perfect for your project and you'll have the trailer side of your project finished in no time.

Too bad wireless is not allowed. Wireless Zigbee like devices are great and in this low data rate application can easily work 10 years on a AA size battery. The advantage of wireless is that you don't have to touch the trailer wiring at all.

[Wayne_]

I also am not quite sure with your description "reverse line" ?

There are many ways you could impliment this using pics.

As stated above, there is the 1-wire protocol which there are drivers/examples already for the pic. This is quite complicated and I would proberbly not use it for pic to pic communication.

Also, take a look at Manchester Encoding, this is also a single line protocol.

Or if your clocks are quite accurate you could just send serial data with local clocking. Start bit, timed data, fixed size or size included in the data stream. You could proberbly use the inbuilt serial comms

The only things you have to sort out is clocking and master/slave communication!

[BOB_SANTANA]

ckielstra

Thanks for your reply and suggestion
sorry if i didn't make it clear

[BOB_SANTANA]

I have being thinking of just having a barcode on the trailer and have a barcode reader in the cab that the driver can just bring out of the lorry when a new trailer is attached read the trailer info then display this in the cab unit then would not need to have a unit in the trailer but i have not used a barcode reader before and do know how complicated this would be.
i like the '1-wire' devices idea are they easy to implement?
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BOB_Santana

[Guest]

The easiest way would be to have a 'database' of trailer IDs and their info stored on a PC.Download the db into the lorry PICs eeprom.I can't believe that there are too many trailers to overflow the storage of the lorry's PIC.
As to 'one wire' communications, I've used it for over 25 years.Mine is a propriatory 'preDallas spec' system that's good for about 30 miles using POTS copper wires.
The barcode is a good idea,1/2 way between my suggestions but 'keeping it simple' should make it more reliable,easier to implement,quicker to program too.

Jay

[dyeatman]

Bob,
Barcodes are the way it is done by many of the larger companies in the US (including the one I work for). All of our trailers have a barcodes and they are scanned automatically by a ground mounted scanner when the trailers enter the yard area. You could mount the BC scanner on the back of the cab and put the BC on the nose of the trailer. The driver would then just "trigger" the scanner to scan for the trailer.

To me RFID would be the best option. Typical RFID systems derive thier power from the transmitter beam so no trailer power would be required. This type system is used extensively by our turnpike systems here in the states.

Some ideas FWIW...
Dave

[adrian]

OK I can see what you are trying to do - but I also foresee lots of problems.

I take it the 'reverse line' is what brings on the reversing lamps and klaxon when the lorry is reversing?
So you are going to have to tap into this wire in the trailer anyway - so some modifications are going to be required to the trailer. You will also have to find the ground wire here somewhere as well.

Next question is how you are going to protect the PIC in the trailer against the 24V when reverse is selected?
Also how do you intend to protect the PIC against load dumps and spikes generated by the tractor's electrical system?

Then we get onto the tractor unit. Now you said that the trailer wiring was not to be altered - I assume that you do not have this problem with the tractor unit, as you said that when reverse is not selected you have to feed the PIC with 5V. I read from this that you are going to have to hack the tractor electrics to switch in 5V when reverse is not selected. If you put in a 5V regulator in the tractor unit what about the voltdrop across the wiring to the trailer? what about noise pickup on this long piece of wire?

I think you will need to go back to your client and persuade him to agree to something else.

[BOB_SANTANA]

Thanks everyone
i have learnt a lot from you all and gone back to him
i emailed him that it has to be wireless modules or the barcode idea its up to him now.

Thanks again for your contributions

Bob
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BOB_Santana

[treitmey]

Reuse wires that are there.
http://www.echelon.com/products/transceivers/powerline/
Data communication over the power and ground wires that are there already.
Rail lines use this as well as Subways
http://findarticles.com/p/articles/mi_m0EIN/is_2005_April_27/ai_n13655864


Slow but robust. Pick up a test kit and see if they talk.
PL Development Support Kit (PL DSK v2.1)
http://www.echelon.com/products/transceivers/powerline/pl31xx_devres.htm#PL_DSK

[SET]

As treitmey says, some form of powerline sounds ideal. You only need communication from trailer to tractor. Tractor couples an AC signal onto the reverse wire which the trailer picks up and rectifies for power. Trailer transmits back onto the line, using FSK. DC voltage on the wire is then irrelevant. Trailer unit just has to find a ground point (anywhere on the chassis).

[treitmey]

There may be a conflict.
http://www.patentstorm.us/patents/7059689.html

[Ttelmah]

Not if he only uses FSK.
This has been done by literally hundreds of products.
The patent is using spread spectrum for the signalling. Existing systems have used FSK, or trellis encoding for higher rates. I built kit doing this over twenty years ago...
I'd suggest first finding out more about the trailer connections. In Europe at least, there are standards for the plugs, and signalling, and some of the newer plugs, have CAN bus already present on the connection. Most older trailers unfortunately, do not have a 'power' connection permanently present, however latter ones do, and this is the easiest line to use. Just add inductors where the powerline leaves the tractor unit, and where it enters the trailer. You modulate the DC line across the link, with a low level AC signal, but because of the inductors, this is not seen by the existing kit on the trailer, or by the tractor electrics. For the low data rates needed for this system, simple FSK will be more than adequate. If required the AC can also be used to power the kit as already outlined.

Best Wishes

[Guest]

There has to be an easy solution to this