Next: Data Acquisition
The NCCS must control the following hardware :
At presently Serial Digital Input Output (SDIO) is used by the
VLBI field system as an instrumentation bus to control some of
the above hardware.
SDIO is a general purpose interface
for doing input and output which can be daisy-chained to make up
It communicates with the main computer via an RS-232 serial link.
SDIO cards have been developed for doing digital input/output and
for doing analog input.
This system has been developed by Justin Jonas of Rhodes University
and has been maintained and adapted by Jonathan Quick at HartRAO.
It suffers from the limitation that if one of the interface boards
goes down then the whole bus goes down.
- select calibration noise diode (8 bit expandable to 12 non-multiplexed),
- master computer fail alarm reset (1 bit),
- select input signals to DVM #1 (4:16 multiplexed),
- select input signals to DVM #2 (4:16 multiplexed),
- select input signals to Pulsar Timer (3:8 multiplexed),
- turn 18cm and 13cm noise adding diode off/on, to be generalised
to include any noise adding diodes (see below),
- select input signal to General Purpose Radiometer (GPR) (4:16 multiplexed),
- select GPR IF filter bandwidth (3:8 multiplexed),
The NCCS must continue to use the SDIO.
The following improvements have to be made however :
No major additional development should be put into SDIO.
Where SDIO is is found to be lacking in functionality Ethernet, HPIB-488,
RS-422 or an industrial solution should be used.
- select input signal to general purpose mixers,
- convert the RS-232 link to RS-422 so as to be able to cover
long distances and be immune to noise,
- group devices together logically on independant SDIO buses to
prevent faults on a single interface from affecting all devices
connected to SDIO,
In addition to the above mandatory hardware to control it is
desirable that the NCCS controls the following
hardware (probably via SDIO) :
Note the old radiometers have not been designed for computer control
and will need to be modified in order to accept computer control.
- configure all receivers i.e.
- select noise-adding noise diode and modulation on/off,
- select dual/single beam,
- select Dicke switch on/off,
- select polarisation mode.
- configure 3.5cm, 6cm, 13cm, 18cm radiometers (e.g. mode is
noise adding/Dicke/total power).
- select input signals to present chart recorders (4 x 3:8 multiplexed)
Next: Data Acquisition