Hi,
Dieter and I have been thinking about building a
simple/basic E1 line
interface. The idea is to simply put trnasformers + LIU + crystal on a
small PCB, which then exposes the Rx and Tx signals at stanard 3.3V CMOS
levels which can be fed in your favourite FPGA or even directly into a
microcontroller.
The first interesting LIU that's available in single quantity is the
Dallas/Maxim DS21348:
http://www.maxim-ic.com/datasheet/index.mvp/id/2848
It can be operated in "software mode" where all configuration (E1/T1/J1,
attenuation, amplification, termination, ...) is set via SPI from a
microcontroller.
However, it expose "Rx Positive" and "Rx Negative" and does not have
the
HDB3/AMI/B8ZS encoder internally. This means there would be two synchronous
serial streams going into the FPGA or microcontroller. This is probably
ok for an FPGA - but it is a problem for most microcontrollers that only
have one sync serial interface like the sam3s/sam7s SSC.
I haven't studied the datasheets in detail yet, but I was immediately thinking of the
Xmos chips (basically multi-threaded 400MHz 32bit DSPs available with a number of
different cores). Several people in the forums claim they have reached clock SPI clock
speeds of 50MHz and they are easy to use (source #paloaltona).
A single chip with one core (xs1-l1) costs USD 8 at Sparkfun, a dual core (xs1-l2) about
USD 15.
The other option seems to be the IDT 82V2081
(
http://www.idt.com/?genId=82V2041E&cid=58553)553), which as internal
HDB3/AMI/B8ZS encoders and decoders (so-called "single rail mode"). It
is a bit more expensive (USD 20 in single qty), but this would enable us
to hook it up directly at a sam3s / sam7s.
As jolly points out, there is still a lot of work required like
framing, s-bits, multiplexing, hdlc, etc. in order to turn it into a
full E1 interface.
But then, maybe there are some applications that don't even require all
those features, such as building a sniffer-only device, or something
that "simply" tries to forward E1 over IP based transports without
parsing too much of the contents.
Open questions:
* stay with 1port design or immediately do a 4port unit? (for
bidirectional sniffing you already need two)
FWIW, I'd do 4 ports.
- Lars