ALMA Real Time

Control System

Jeff Kern

Ralph Marson, Thomas Juerges

Real Time Control System

• Real Time Control is the portion of the system which has fixed timing requirements

• ALMA uses a 48-ms timing pulse to synchronize commands across the Array– Synchronization below the 48-ms level is

done in hardware

Real Time Subsystems in ALMA

• Fringe and Delay Tracking– Distributed: Done partially at the Antenna (64) and

partially in the Correlator (2)

• Antenna Pointing– At least 66 Antennas

• Data Acquisition– Correlators (2)– Total Power (Square Law) Detectors

• 2 kHz / Baseband / Polarization / Antenna

– Holography Receiver for Beacon Holography

Hardware Architecture

Array Operations Site (AOS)

Operations Support Facility (OSF)

OperatorConsole(Master)

ArchiveOnlineProcessing

Antenna Bus Master(ABM)

HardwareDevice

48- ms Pulse

Array Real Time Machine

(ARTM)

ALMA CompactArray Correlator

12-M ArrayCorrelator

HardwareDevice

HardwareDevice

Ethernet

ALMA Monitor and Control Bus

(AMB)

Software Architecture

• Most Control systems for large telescopes have the same high level architecture.

Master Archive

WeatherStation

Back End Front End Mount

Software Bus (Middleware)

ALMA Monitor and Control Bus (AMB)

• Controller Area Network (CAN)– 1 Mbit / s Multidrop Serial Protocol– 35-m Bus Length with up to 32 Nodes / bus– 29 Bit address scheme

• AMB adds to the Controller Area Network (CAN) standard.– Master-Slave architecture to prevent collisions on the

bus (provides determinism)– Broadcast protocol (device discovery)

• Timing Event • RS-485 Reset Pulse

November 17-21, Santiago 6ALMA Computing Review

Software Design

• Local Control Units (ARTM, ABM, etc) are Linux systems with a real time kernel– Real-Time code is written as kernel modules– Minimize real time code

• Timing Event interrupt handler• Real time aware device driver

• Non-Deterministic Ethernet between master and real time computers.– Commands are queued in advance (~1 sec) and are

executed by the interrupt handler.

Lessons Learned

• A larger middleware does not necessarily produce smaller device interfaces.

• Modern Programmable Logic Devices (FPGAs) are often a better solution than general purpose real time computers.– Better Latency– Integrated with the hardware– Allow true parallel behavior

Is this the End of Real Time?

• Some applications still require general purpose real time computers:– Data Acquisition– Real Time Data Processing (Adaptive Optics)– Distributed Servo Systems

• Small projects without a VHDL expert and no resources to invest in developing one.