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"Teslink" multiplexed fibre optic link

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Steve Conner

Thu May 01 2014, 10:31AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi all

You might be interested in my new project. It uses two 18 series PICs (programmed in old-school assembly

) to transmit multiple channels of data down a single optical fibre, for Tesla coil control.

The data consists of a single interrupter channel sampled at 200kHz, plus a subcode data stream running at 50kbit/s in a format somewhat inspired by DMX, that can encode any other data you want. I plan to use this to send a couple of analog voltages, which will be sampled by the transmitting PIC's ADC and output by the receiving PIC's PWM channels, and some general purpose digital lines.

The prototype setup uses two PIC18F4431 processors clocked at 40MHz. It shows a latency of about 120us, and a jitter of 5us in the interrupter signal, as would be expected from the 200kHz sampling rate. The scope shot shows a 5us pulse being transmitted.

The data on the line is currently 1MBit/s, Manchester encoded to avoid long runs of 0s or 1s, so AC coupled Toslink receivers can be used.

The system allows for various kinds of parity and error detection, but I haven't finished all that code yet.

When the code is done I will release it as open source.

Mads Barnkob

Thu May 01 2014, 10:42AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Interesting that you took on this project, I have thought about using digital IC MUX to do the same, however much cruder.

Will it be able to transmit data out from the coil? so we could monitor temperatures, voltages, currents etc?

Steve Conner

Thu May 01 2014, 10:51AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The current system only transmits data in one direction, as it is designed around a single fibre. There's no reason you couldn't use two copies of the system though.

This is intended as a simple, low-cost system using only through-hole components. The cost difference between the Toslink optics and the HFBR series more than pays for the PICs.

I have plans for a "Teslink-2" using gigabit SFP transceivers, which can be found quite cheaply on Ebay. These could be connected to the Dallas DS90UR241/124 serdes chipset to give 24 completely general purpose digital lines sampled at 4MHz, or hooked up directly to the FPGA in a future FPGA-based driver. This would be a serious project needing 4 layer boards and SMT assembly, though.

HV Enthusiast

Thu May 01 2014, 12:50PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Great project.

I've actually done this before and use it pretty extensively, although i didn't use it for Tesla Coils.

I use it for transmitting control and analog data to a high voltage floating deck which contains filament and grid power supplies for high power microwave tubes. The filaments and grids must be monitored and controlled via fiber optic, as the deck is floating at anywhere from 40-60kV, so this was a way of doing it without using a single fiber for every single signal.

I used standard Agilent HFBR fiber optics.

Keep us posted on your efforts!

Steve Conner

Fri May 02 2014, 05:02PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

A bit more progress to report:

Implemented some error checking including parity, to drive a "data valid" LED and set the outputs to a failsafe state if anything goes wrong.

Hooked up to a 15m Toslink cable and had some fun testing the error detection by pulling the cable out of the receiver.

Implemented the subcode reception state machine.

Successfully transmitted the ASCII string "Tesla" over the subcode channel.

Next week I'll get the subcode channel working with live data from the ADCs and the programming will be more or less finished.

Steve Conner

Fri May 09 2014, 12:12PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

As promised, the code!

This version currently transmits the interrupter signal and two 10-bit analog values, which are reconstructed as 2.4kHz PWM at the receiver. Feel free to hack away! It could do with some general-purpose digital outputs.

]teslink.zip[/file]
]teslinkrx.zip[/file]

Thomas W

Fri May 09 2014, 04:19PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

I have som interest in fibre optics. My main thing is i can't find any good places to find the fibre optic cables, recievers, transmitters or trancievers.

Any good pointers?

And nice work steve!

Steve Conner

Fri May 09 2014, 04:39PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Thanks!

I always got my fibre optic cables on Ebay, and the transmitters and receivers from Farnell.

For this project, the 15M cable cost Â£10 and the receiver and transmitter were about 75p each.

Thomas W

Sat May 10 2014, 11:16AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Thanks Steve, i found the ones you are talking about now.

Also had a look at the SFP transceivers
Very cheap and interesting really, as it happens each module works out around the price of a pair. maybe a bit more when used and from ebay...

One thing i did notice, is that they should be mounted into a frame on a PCB.. where would you get somthing like that? I searched high and low online with little to no sucess.

Steve Conner

Sat May 10 2014, 11:55AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The SFP transceivers need a LVDS input. Not much use without something like a FPGA or the Dallas chips I mentioned.

You can get the SFP "cages" from Digikey.

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