The saga of the CupreDSU

The CupreDSU was probably the coolest SDSL CPE device ever made. It was nothing less than a true bona fide DSU for SDSL! CupreDSU was not a router or a bridge, just a DSU. It had no Ethernet port and no IP address. The unit's only connectors were for power, SDSL, V.35 (seems like it was EIA-530 in reality) and a craft terminal port, and it appears that the unit truly was a completely transparent bit stream access device. The product's only advertised DSLAM interoperability was Copper Mountain.

The CupreDSU was built by Larscom, a company which exists no more (devoured by Verilink). However, the product was discontinued long before Larscom went out of existence; in fact I was told by Larscom that they had killed the CupreDSU after only about 500 units were made, making the article absolute pure unobtainium. The only thing left of it is its manual. (They said that no one was buying it — goddamn market crapitalism.) When I had first heard the above, I was absolutely devastated: I wanted a CupreDSU so badly and didn't know what I know now regarding its triviality (see below).

Copper Mountain SDSL was still shrouded in mystery for me at the time, and I thought that Larscom had done something very slick in their DSU design to make it work. I also assumed that the design contained some crucial proprietary magic from Copper Mountain. Thinking that it was a very important and non-trivial design that had to be brought back from the ashes at any cost, I had told Larscom that I wanted to buy the manufacturing rights to their unsellable product. Larscom which still existed at the time was receptive to my idea, but the problem of course is that I'm just a person of average means, not a rich capitalist pig, and my work is strictly non-profit. Care to guess how much I had offered Larscom for the manufacturing rights to their CupreDSU? It was $1000, a little less than double their list price for a single CupreDSU unit. I had told them that it was still better than the absolute zero they were making otherwise from their dead design...

I don't know whether or not the person at Larscom I was talking to fell out of his chair laughing after he got off the phone with me — perhaps we'll never know. I can't just ask him or his wonderful assistant because neither of them is employed any longer by Verilink, and I have no way of reaching them. However, I have since found something else that changes the picture significantly.

My desperate desire to bring the CupreDSU back from the ashes by getting the schematics, gerbers and manufacturing rights from Larscom was based on my assumption that the design contained some special proprietary magic from Copper Mountain or other non-trivial stuff that would be very wasteful to recreate from scratch. But now knowing everything we've come to know through this Open SDSL Connectivity Project, we know that this is not the case.

Replacement

As we now know, Copper Mountain SDSL is nothing more than SDSL Flavor B, and the mysterious bitpump is nothing more than an off-the-shelf chip from Brooktree/Rockwell/Conexant used in the straightforward manner. In light of these observations it becomes evident that the CupreDSU is not as special and irreplaceable as I had originally thought.

It is now evident that a Flavor B DSU like the CupreDSU is actually an absolutely trivial design: it is literally nothing more than an RS8973 bitpump whose RDAT and TDAT pins are connected to a synchronous serial DCE port with a little microprocessor to run the standard BT software for controlling the bitpump. The CupreDSU will thus be completely superceded by our OSDCU. (Being originally intended as a debug and reverse engineering instrument rather than an end user appliance, the OSDCU has some extra features not needed for DSU operation: the SCCs, the optional FPGA, and a microprocessor system probably much more powerful than necessary. But hey, it's a fun toy!)

If anyone else misses the CupreDSU, help us finish the PCB layout of the OSDCU and the replacement DSU will be ready!

Curiosity

Completely aside from the practical considerations of functionality replacement, there is one thing I'm very curious about. It seems from the CupreDSU manual that the beastie has used a far more powerful processor and firmware architecture than necessary.

Let's start with the serial console port. A dumb DSU like the CupreDSU doesn't really need one and indeed according to the manual, this port wasn't used for configuration. Normal CM CPE operation required no configuration at all, and the campus modem mode was configured with 4 DIP switches and an internal jumper. Instead the only things one could do through the serial console port were displaying a little status screen and downloading firmware updates.

However, the description of console port operations in the manual mentions two things that make me very curious:

The questions which arise from these observations should be fairly obvious:

For comparison, consider how equivalent SDSL DSU functionality could have been built from Conexant's evaluation platform for the RS8973 bitpump. That platform used an 8051 toaster controller as its processor, and could have been turned into an apparent CupreDSU equivalent with the following simple modifications:

  1. Take an RS8973EVM system without the channel unit card.
  2. Replace the BNCs with an EIA-422 transceiver for an EIA-530 DCE port.
  3. The provided sample 8051 firmware already reads two DIP switches to select the data rate. Add a third one to have 8 speed choices like on the CupreDSU.
  4. (Optional) If desired, implement the code to receive and count CM speed code pulses, overriding the DIP switches.
  5. Done!

In contrast, Larscom appears to have used some sort of full-blown RTOS architecture. I wonder why. Could it be because they disliked the 8051 as much as I do? :-)