Historical interest only: unchanged since February 2000
While I provide a list of plausible chips below (if the boards were to be constructed today), the point is to generate an interface that can last even as chips come and go. The test for this (and any other) architecture is, can one plan to rebuild any given board at any time in the next 25 years, with whatever technology is on-hand at the time? Part of this question is, are the interfaces fully documented, and relatively simple, so we can expect that implementations will be bug-free?
(eps copy) Each blue rectangle represents a circuit board, more on the construction below. The two rectangles on the left are repeated a few times per computer, mostly determined by how cables conveniently run. 6 copies are appropriate for one MEBT raft. Less than 4 doesn't sound cost-effective. More than 10 would probably swamp the backplane, computer, and network.
Specialized front ends:
Because of the JTAG connection, the backplane and its connection to each FPGA is fully in-place testable. However, an FPGA with a broken JTAG port (or a missing board) keeps the rest of the crate from being programmed. JTAG files to support each specialized front end are kept on the 'net. The Flash memory on the computer is only enough to boot.
Normally, only heavily averaged data is transmitted from the FPGA to the computer, after each beam pulse. A snapshot of raw ADC values sticks around in RAM, so a history can be regurgitated on demand. The computer does the complex manipulations like linearization, before passing output to the Ethernet port.
The computer does not necessarily run EPICS. If not, it would spit out regular UDP packets containing the per-pulse summary information to an EPICS computer, which would then gateway that information to the rest of the net. The EPICS computer would also necessarily relay some tcp connections for more intimate debugging, and possibly the raw waveform dump function mentioned before. This mode has the advantage that the CPU horsepower needs are lower, and the net bandwidth needs are lower. I haven't seen embeddable 100 MB/s Ethernet subsystems, while 10 MB/s Ethernet is widely available in single-chip form.
The ADC/FPGA board is intended to be common across the specialized front ends, but gets mated and tested with a front end before insertion in a crate. Both the ADC/FPGA and Front End boards should have a form factor about 10cm square, but a little bigger is OK. 10x20 cm is more than big enough for the computer.
I imagine a crate holding all these cards tucked in close to the beamline in the MEBT. I am told this thing needs to be waterproof, so the cableway needs a cover with a foam rubber gasket on the bottom where the cables come out (see the next paragraph). Add 1cm on the bottom for LO and Calibration splitters, and 4 cm cable space enclosed by the cover, and the total box is about 11x12x25 cm. Note that the power consumption of all this probably tops out at about 1 Watt per board, so conduction cooling should be plenty. No fans! Board count for 1 MEBT raft:
Besides the dozen or so .141 hardline connections to the beamline, there are the following connections to the control racks:
I'd like to see all this bundled up, maybe with spiral wrap. It looks like two .141 hardline, at least one CAT5 cable, and a (low voltage, thick wire) power cable.
I have not made provision here for a remote 'scope connection (multiplexed) to the Front End signals. At JLab, such a thing is called an "Analog Monitoring System."
I would like to see some status lights, like
I don't have a good plan to make lights that are visible from a distance, protected from water, don't have a lot of hand wiring, and are compatible with quick-change cards in a backplane.
January 19, 2000