Hi 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 <[email protected]> http://laforge.gnumonks.org/ ============================================================================ "Privacy in residential applications is a desirable marketing option." (ETSI EN 300 175-7 Ch. A6)
