SCUBA-2 CoDR: Electronics and data acquisition: Page 1 of 3
Electronics and Data
Acquisition System
Contents
1. Introduction
2. Electronics overview
3. Data acquisition options
1. Introduction
In this section we summarize the requirements of the post-SQUID and multiplexer
electronics, which includes the design of the data acquisition system.
2. Electronics overview
ELECTRONICS FUNCTIONAL DIAGRAM
0.5K 4K 300K
SQUID BUFFER MULTIPLEXER
ARRAYS AMPS
FPGAs
hυ A/D
VME
system
TES’s Feedback D/A BIAS
Bias
Reflector
chip/Buried Address lines
SQUIDS
JCMT
CONTROL AND
COMPUTING
SYSTEMS
SCUBA-2 CoDR: Electronics and data acquisition: Page 2 of 3
3. Data acquisition
The transputers used in SCUBA are now obsolete, and it is no longer possible to purchase
any parts for Inmos T800 transputers. In addition, the new TODD observatory driver
system, is due to be released within a year and the SCUBA transputer system does not
‘speak’ the same language as that being adopted for the new TODD, VxWorks/Drama
and Unix/Drama systems. It has also been proposed to install a fibre-optic fast data link
for the current SCUBA system, and this is awaiting approval as part of the SCUBA Phase
2 upgrade programme. The current system is only able to transfer full-speed data from
the Long Wave array continuously. This severely limits fast scan modes, such as the
recent DREAM experiment. Clearly, transputers are not a viable option for SCUBA-2.
3.1 Proposed schemes for SCUBA-2
We are currently considering two schemes for SCUBA-2 data acquisition:
3.1.1 Scheme #1
This scheme is essentially summarised in top-level form in the electronics block diagram.
The multiplexor contains 14-bit A/Ds and we use link adaptors of a similar design to
SCUBA. This is shown in the schematic diagram below.
Link adaptors
A/D
VME digital I/O card
8-bit wide control/status
FIFO buffers
A/D
VME CPU card
A/D
16-bit wide data
PMC DMA I/O
Multipl
MULTIPLEXOR
exor
A/D
In this regime, the old SCUBA transputers are replaced by a VME VxWorks/Drama
system which simply collects the data and sends it back to a workstation for processing.
Existing processing algorithms that are used in the transputer system are currently written
in C and can be ported to the another workstation.
SCUBA-2 CoDR: Electronics and data acquisition: Page 3 of 3
The VME system would include a specially designed VME board which basically
consists of transputer link adapters and FIFO buffer chips. The link adapter chips could
receive signals from the multiplexor every 7.8 milli-seconds at a burst rate of about 1
Mbyte/sec. The FIFO buffers would slow this down to the average rate of 3.7 Kbytes/sec
which would then be direct memory accessed (DMA) in over the VME bus to the micro.
The PMC interface shown in the diagram is the same as the one that Michelle will use,
though any one of many could perform this function. A standard digital I/O card is used
to control the A/D system as well as the link adapter VME board.
3.1.2 Scheme #2
This scheme takes #1 to its fullest potential. The multiplexer is simply absorbed into the
VME system with the signals from the buffer amps going straight into the computer. The
challenge would be to design the VME system so that all the multiplexed signals could be
accommodated. This is similar to the design for the RxH3 micro. However, this board
only has a 12-bit A/D, but it should be possible to get higher resolution cards.
The advantages are that the need for electronics/electronic design is virtually eliminated
and the SCUBA-2 system would wholly consist of industry standard, off-the-shelf
components. There is a price to pay of course, literally. These boards are typically
$2000 each, though this would be somewhat less with quantity discount.
There may be a problem worth housing the A/Ds inside the computer system in respect of
electrical interference. It can be assumed that with the modern VME systems this is more
of a known and controllable problem.