I've started re-reading Matthew Gast's T1 - A Survival Guide. The last time I read it, I was simply looking to answer some specific questions while attempting to connect my Asterisk box to my NEC NEAX 2400IPX PBX at my work. (BTW, I was able to successfully connect my Asterisk and NEC PBX's using the quad T1 (T400P) card I have - both with 'line side' and 'trunk side' T1 cards in the NEC.) This time around I am trying to get a deeper and more complete understanding of the T1 spec. Fortunately, I've been a 'telephone guy' for about 12 years, so the telco jargon doesn't affect me anymore.
I have to give Gast two thumbs up for his book. It is shamelessly focused on American telco and history, but since I'm in California I don't mind. Besides, many of the principles apply on a broad scale, and the specifics of E1's (Europe) and J1's (Japan) can be gleaned from other sources. I have no intention of being Yankee-centric on this project - I guarantee that I will get as much help from around the world as I will domestically. (In fact, the only person to contact me so far is not from the Western Hemisphere.)
Thus far I feel comfortable with the basics of the T1:
I have also started digging into the specs of what I suspect will be the IC that is used for T1 communications: the Dallas Semiconductor DS2155. This is a single-chip transceiver (SCT) that seems to have lots of capabilities. The quad version of this chip is what drives the T400P boards that I currently use with my Asterisk boxes.
I printed out all 238 pages of the data sheet, plus a nifty application note about creating a circuit to connect to the telco and how to protect my circuit from voltage spikes, etc. The data sheet has all sorts of detailed information that I believe will be useful once the firmware and device-driver stages are being considered. I'm reading it now because I'm hoping to find any red flags and address them before I get too far into the project. If this chip is not suitable for the project then I need to know ASAP so that I can go elsewhere. If anyone has any experience at all with this or similar SCT's or T1/E1/J1 framers, by all means send me your feedback and advice.
I've learned a few interesting things about T1's so far: timing is inferred from the bit stream - an interesting concept. (Yes, there are LOTS of timing and clocking factors but I don't have time or space to list them all.) Also, the time slot for a single bit is 684ns. An entire extended superframe (ESF) has 24 8-bit frames for a total of 192 bits. That's a little over 131 microseconds for an entire frame to be read! Yeah, that's fast. It makes me wonder about how easy (or difficult) it will be to get the datastream to and from the USB controller without messing up the clocking. We'll see.
I have ordered a pair of DS2155's (free samples) from Maxim for use in my 'lab' - such as it is. By lab I mean "a cubicle at work that no one is using at the moment." Still, it's clean and quiet and the boss lady doesn't mind as long as I get all of my regular work done. When I get my units I will then start looking at the other fun stuff of this project: PCB's, FPGA's, USB Controllers, various components, etc. In the meantime I've been perusing the spec sheet of the DS2155 in an attempt to familiarize myself with its capabilities. I've also been looking at the original Zapata telephony Tormenta2 specs since it uses a similar SCT, the DS21Q352. (The T400P's that I have are very similar but they use the slightly more updated DS21Q55 - the quad version of the DS2155.) One thing I appreciate is that Jim Dixon has lots of schematics for viewing, which is cool because it helps me visualize the flow of data from the smart jack to the PCI bus and back. While this isn't totally analogous to the flow of data to/from the USB controller, it is a good jumping off point.
The next postcard will have an update on my (lack of) progress with gEDA, plus some research into the world of PCB fabrication.
-MC
Summary:
- I'm re-reading T1 - A Survival Guide to fill in the mental gaps
- I'm happy to report that one person so far has helped me with the project
- I've printed out all 238 pages of the DS2155 spec sheet, plus app notes from Maxim
- I've been looking at the original Zapata Tormenta2 specs for education, inspiration
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