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Harald Welte laforge at gnumonks.orgHi all,
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.
The other option seems to be the IDT 82V2081
(http://www.idt.com/?genId=82V2041E&cid=58553), 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)
* how do we achieve synchronization between Rx and Tx data path?
Looking forward to your comments, feedback.
Regards,
Harald
--
- Harald Welte <laforge at gnumonks.org> http://laforge.gnumonks.org/
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