It would be simple to suggest something real if you tell me something
more about your constraints.
The application is a professional video product intended to interface
encoder signals off a camera system with graphics equipment at a
different location. Many camera systems generate various information for
their own internal use (pan, tilt angles, boom position, etc) and there
are several specialized outboard applications that piggyback onto this
information.
The receiving end expects data in this parallel form, or else I’d just
parse it out at the source and transmit the positions down a
conventional serial line.
The systems currently in use are based on 12 pair cables driven via
standard differential techniques – a simple parallel bus. This has
disadvantages, of course. The obvious one is the expense, size and
fragility of the cables, but a secondary effect is the unsuitability of
DC coupling in large systems.
Though there’s a surprising amount of signal conditioning involved you
can consider the signals to be TTL quatrature encoder signals, plus some
low-speed sync signals. Nothing above 100 Khz. A 1 Mhz sample rate is an
arbitrary rate that gives me reasonable duty cycles and timing precision
given a squarewave in at 100Khz, plus a little room to grow, just in case
The data is essentially a 24 asychronous bits, sampled periodically, and
reconstructed at the far end.
Ideally, I’d like to have a solution that’s the functional equivalent of
the commercial serializer/deserializer chips (see the Cypress HotLink or
T-I MuxIt parts). You latch data into the transmit side, and it pops out
the receive side some microseconds later.
The transmission line has to be 75 ohm coax, because it’s ubiquitous in
the industry. I can’t use differential methods - one line is all the
clients will tolerate. Transmission distance is now typically no more
than 30m, but I wouldn’t be happy unless I could guarantee at least 100m
because someday somebody will want to do that.
It has to be an AC coupled system, to avoid system problems involving
ground loops.
Though I can be pretty sure they won’t use really cheap coax, I likewise
cannot guarantee that they’ll use really good coax. If I spec, say,
Belden 8281, that spec will likely be ignored. Consequently, I have to
worry about noise issues, both transmitted and received. The environment
tends to be electrically noisy because of high frequency lighting equipment.
Despite all the money video productions spend, like everything in the
accessory segment of the pro video market this product is surprisingly
cost sensitive. Therefore, I’ve been trying to stay away from the
commercial ser/des chips.
I’m wary of part cost (the chips themselves, an additional 3.3V supply,
and the handful of gates and level shifters at either end to pack and
unpack the data into 8 bit words) I’m cautious about the need for a
better medium than I have (the parts typically clock at 160Mbs and up)
and that the parts are mostly available only in surface mount. There’ll
be small production runs (<100 at a time), and I hate to add just 1 SMD
to an otherwise all thru-hole board.
Some quick calculations seem to show that an FPGA at either end could be
cheaper, even when you consider handling and programming, but there’s
the design time to amortize (I haven’t started full time this project
yet, it’s up next)
Power is not an issue, there’s +5V at either end for the rest of the
logic. I could add another supply, but I’d prefer not to.
Though I’m no stranger to digital design, I work mostly in
microcontrollers and motor control, and communications is enough of a
specialty art that I know when to ask for hints and tips.
Got any?
- Steve