An updated version of the VSI-H draft is attached in both ps, pdf and Word format (take your pick). This draft has incorporated what I feel are the best choices on outstanding issues based on comments I have received (and distributed) plus a a measure of my own best judgement. Any or all of these issues are up for discussion at the 04 May 2200UT telecon.
Below I have relisted each of the issues and indicated how I have addressed them in the attached draft. Your comments are, of course, welcome.
1. LVDS Connector Pinout
Issue: Dick Ferris has suggested a pinout different from the 9 Feb draft document. This new suggested pinout is based on the use of a specific LVDS device. The Japanese group have already begun implementation of the 9 Feb pinout.
Discussion: Although Dick's is more optimal for a specific LVDS interface device, the 9 Feb pinout has been chosen to be generically 'good' in a general engineering sense, is likely to be workable, and does not constrain the use of future LVDS interface devices. The fact that the Japanese group has already partially or fully implemented the 9 Feb pinout makes a change painful.
New Draft: I have been won over by Dick's arguments in his 3 May e-mail. Though I know this may not please everyone, I think the long term benefits are clear. Please read Dick's e-mail carefully.
2. Bi-directional LVDS Signals
Issue: In his notes of 3 Mar 00, Dick Ferris has suggested the use of reverse-channel functions. The 9 Feb draft specifies only unidirectional signals on LVDS cables.
Discussion: The possible use of bi-directional signals on the LVDS cables was a *major* discussion item at the Haystack meeting in late January. In the end we agreed that the signals on each LVDS cable would be uni-directional. That is *the* reason that DPS1PPS and DPSCLOCK are on a separate cable to the DOM. Despite the fact that reverse-channel signals do provide some possible advantages, I think we must stick to the January agreement for the base VSI specification. As Rick points out in his note below, a reverse-channel capability could still be built into the DIM/DOM using bi-directional driver/receiver chips, but that they must be configurable to meet the uni-directional VSI-H specification.
New Draft: I apologize that I did not fully understand Dick's proposal to make bi-directional signals entirely optional in a manner that does not in any way violate the uni-directional decision of the January meeting. I have since studied his proposal much more careful, believe that it is good, and have basically incorporated it. Please read this part of the draft very carefully.
3. Delay in DOM/DIM
Issue: A delay option in the DOM was suggested in the 9 Feb draft after some discussion at the January meeting. It was probably my fault that it was vaguely defined, which left the issue rather unclear. Since then, there have been various opinions expressed regarding its implementation, and Rick suggests adding a 'delay' option to the DIM, as well.
Discussion: Adding a fixed delay capability is clearly useful to help in the case of limited size of correlator input buffers. Anything fancier, such as delay tracking, has very correlator-specific requirements that are very difficult to accomodate.
New Draft: For *storage-based* DPS systems (i.e. tape, disc, any transportable media), the required delay range is +/-0.5*ROT1PPS. Dick points out, correctly, that for *direct-transmission* systems, the required delay is highly dependent on antenna-location geometry and could place undo requirements on a DTS system; I feel that, in this case, that VSI-H mandate no delay-offset requirements and assume that the DPS will implement whatever is necessary.
4. Bit-stream selection and re-ordering
Issue: We have agreed that any 2**n input bit-streams at the input to the DIM can be selected for transmision to the DOM. In addition, we have agreed that arbitrary bit-stream re-ordering at the DOM output is necessary. Dick Ferris has also suggested re-ordering in the DIM.
Discussion: The benefit of re-ordering in the DIM is not clear to me. Arbitrary bit-stream re-ordering at the DOM output is sufficient to cover all bases. The 9 Feb draft comments that DOM-output re-ordering can be effectively achieved in a separate box between the DOM and DPS. Dick Ferris comments that a separate box makes it difficult to define a standard software protocol.
New Draft: I feel the only real requirement is that any active DIM input can be mapped to any DOM output. And that is it. How that is implemented is up to the designer. Re-ordering at the DIM might add some features and conveniences, but I don't think it belongs in the specification; I did add a footnote in Section 13.4 in this regard.
5. LVDS Cable Specs
Issue: The detailed specification of cable is very complicated.
Discussion: Dick has done a very admirable job of creating a creating a suggested cable spec, but the Japanese point out the difficulties of making objective measurements of some parameters.
New Draft: I have lifted the basic cable specifications from one of Dick's memo's and made some general remarks. There may still be a bit of work to do here if people feel it is incomplete, misleading or inaccurate.
6. LVDS Electrical Specs
Recommendation: Adopt the detailed specification given by Dick. This is more detailed and complete than the 9 Feb draft.
New Draft: Done
7. Validity per bit stream
Issue: The 9 Feb draft proposed a extension using bi-phase code for validity per bit stream. Dick Ferris points out that this introduces many complications.
New Draft: The whole validity-per-bit-stream issue has been relegated to 'Other Notes and Comments' so that we don't forget about it, but there is nothing in the spec about it.
8. Serial control port spec
Issue: The 9 Feb draft proposed is not fully internally consistent in the specification standards.
Discussion: Dick makes the point that RS-223 is probably not the proper spec. Count me guilty in that regard. At the January meeting we had agreed to use RJ-45 type connectors with the pin-out specified in Table 12 with only XON/XOFF handshaking specified. Dick points out that the specified pinout does not follow the relevant EIA-561 standard, and that the specified pinout is bad in terms of crosstalk when using standard CAT-5 cables. The problem is that none of the major manufacturers (Cisco, Lantronix at least) use the EIA-561 spec. They use the Table 12 pinout in spite of the problems mentioned by Dick; the advantage of this pinout is that its trivial to create either a straight-through or null modem cable by simply flipping over the RJ-45 connector on one end. Dick also suggests allowing DB-9 and DB-25 connectors as well, plus RTS/CTS flow control.
New Draft: Consensus seems to be to adopt DB-9 rather than RJ-45. This both eliminates any possible confusion between the RS-232 and Ethernet connectors and puts the pin-out issue to rest (I trust!).
9. Proposal for P/QDATA format
Recommendation: Leave this issue for VSI-S spec
New Draft: Done
Issue: The example TVG circuit diagram may not exactly reflect TVG intentions and may cause confusion.
Discussions: The TVG timing relationships are specified in Figure 4. If Figure 4 is used a guide along with the TVG diagram and the listing of the first few bits of each TVG output, I hope that things will be sufficiently clear.
New Draft: I have tried to clarify this area.
11. Test Vector Receiver
Issue: Whether TVR should be able to 'untangle bit-stream mix-ups'
Discussion: Dick points out that untangling bit-stream mix-ups has no role in ordinary testing of interfaces since there is no mechanism for bit-stream mix-ups to occur, and that a significant piece of hardware would be necessary (in particular, a 32*32 correlator) to fully meet this requirement. If there is a mix-up, it can be easily untangled by examining the first few bits of each sequence and comparing against Table 13.
New Draft: No requirement, but a footnote to the effect that this capability might be nice.
12. LVDS Receiver
Issue: Dick Ferris suggests that a receiver with no input should fail-safe to logic '1'. Other suggest that the fail-safe state should simply be a stable '0' or '1', with no preference.
New Draft: Fail-safe to logic '1', which is actually part of the TIA/EIA-644 standard, as Dick points out.
13. TTL Monitor Ports
Issue: The 9Feb draft specified TTL monitor ports should be designed to drive into 50-ohm load. Dick Ferris suggests that, if the ports are only for occasional scope monitoring, that a better way is to use a 50-ohm 'back termination' by placing a series 50-ohm resistor on the TTL driver.
New Draft: 50-ohm back termination. We can argue about this if you want.
There are probably some other things that I have changed and forgotten to mention, but none major, so it would be well to read the whole draft carefully. In a few places I have re-organized for clarity or logical structure, but you can judge whether these changes have the intended effect. The VSI-H document has grown to rather substantial size, as you can see, and I hope that it does not grow any larger. As you know, I am very anxious to get *all* of these issues settled soon and move on to other things (VSI-S?). The longer we drag things out the less useful the VSI-H spec will be.
Early next week I will e-mail the details of the 10 May telecon. Please explode any comments you have directly to the distribution group of this e-mail because there is little time for me to receive and explode them. We may all have to give and take a bit to come to final agreement, and I trust that everyone is willing to be a flexible as possible to nail this thing down and get it done.
Regards and thanks, Alan