The GBT is a collection of mechanical and electrical subsystems, which, taken together, form a radio telescope. The coordination and control of these subsystems is accomplished using M&C system, a hierarchical distributed system consisting of software modules running on multiple computers communicating over a local area network (LAN). The M&C system is implemented using general-purpose computers as well as specially designed hardware. The system is designed to minimize real-time communications and timing dependencies between subsystems. This allows the asynchronous Ethernet protocol to be used successfully for control purposes and for data transport. The whole system is synchronized via a combination of Network Time Protocol (NTP), InterRange Instrumentation Group B (IRIG-B) and 1 Pulse Per Second (1PPS) signals distributed by the site timing center.
Throughout GBT documentation we see references to "VME" computers and hardware. To bring some clarification to this aspect of hardware, I would like to present the following account as explanation of what VME stands for:
What meaneth VME?
In 1981, when Motorola agreed to allow Mostek and Signetics to second source the MC68000 microprocessor chip, the three companies agreed to meet and discuss the possibility of supporting a common backplane bus. At that time Motorola had already developed a 68000-based backplane bus, which they called VERSAbus. Since I had written a large portion of the VERSAbus specification, I was the Motorola technical representative at that meeting.
Motorola proposed that the three companies jointly support the VERSAbus backplane. However, both Mostek and Signetics rejected that proposal, saying that the VERSAbus board size was much too large. In response to that objection, Motorola proposed that the (much smaller) Eurocard board size be used. A backplane could then be designed with the VERSAbus electrical specifications and the Eurocard mechanical specifications. Motorola suggested that new board products (based on this new backplane) be called VERSAmodule Eurocards, which could be abbreviated "VME".
Both Mostek and Signetics were satisfied with the choice of the Eurocard mechanical standard, but they objected to the "VERSAmodule Eurocard" name, since Motorola had already trademarked the name "VERSAmodule".
Eventually this difficulty was overcome when the three companies agreed that the name VMEbus would be placed in the public domain, and that if anybody asked what VME meant; they would say....
"VME?...Oh, it doesn't stand for anything in particular".
Now you know the awful truth.
by: John Black
Editor
VMEbus Systems Magazine
Tempe, Arizona
Data storage on the GBT is accomplished via a 50-75 GB RAID system. The RAID system is a set of hard drives physically interconnected and operated with special software drivers. The drivers allow data to be written to one drive and backed up on a successive drive in the RAID set. By doing this, data is continually archived on a drive other than the one it is stored on. In the event of a drive failure, the data can be quickly recovered by just replacing the drive. The data is reconstructed from the data archive off the redundant disk.
With 4 host ports, the RAID set is capable of writing data at 60 Megabytes per second. At such a high rate, up to three back ends can write to the RAID set at a time. VLBI data is stored directly on VLBI tapes by the S2 rack or the Mark IV tape drive.
Data storage is complimented by the use of data tapes and CD ROM Writers for long term storage and off-site transport of data.
The telescope operators utilize screens displayed on multiple monitors to control and monitor the telescope. These monitors are standard 21 inch Sun monitors, with a resolution of 1152 x 900 pixels.
Data is collected and written to the RAID via any one of three computers. The spectrometer and spectral processor each contains its own data collection machine, and an additional data collection computer is used to aggregate the data from the rest of the telescope systems. A fourth computer is connected to the RAID for read-only access to the data via the Network File System (NFS). It is located in the equipment room and serves as an engineering workstation, to be used mainly for troubleshooting and working with the equipment in the equipment room.
The Precision Pointing System uses another workstation as a compute server, and coordinating the tasks of the Laser Metrology system and the Active Surface system. This computer is located on the telescope allidade, approximately 2.5 km from the control room.
Closer to the machinery, VME based computers contain the interface hardware, and run most real-time tasks under the VxWorks operating system. Hardware interfaces are provided to connect the telescope servos and instrumentation to the computer systems.
The real-time needs of the telescope are met using Motorola MVME-167 68040 based single board computers. Twelve of these run M&C code, while 2 others are supplied with the contractor's Antenna Control System.
Several other VME cards are used in the system including the Bancomm IRIG decoder, parallel I/O cards, IEEE-488 interface cards, and analog to digital converter cards. These devices serve to connect data streams to the M&C system. All the chassis used in the telescope have at least three slots open for future expansion.
The Bancomm BC-635 card is used in each chassis to receive and decode IRIG signals from the site timing center, and synchronize the chassis to the site clock.
The parallel digital interfaces are Motorola MVME-340 cards. The cards have software written for them that allows the higher-level software to read and write bits on the card without having to program all the registers on the interface chips. The inputs and timers on these cards are used to generate interrupts for synchronizing the systems in the telescope; for example, the Tracking Local Oscillator (LO) can switch frequencies on the edge of a signal/reference (sig/ref) or calibrate (cal) signal.
The Mizar MZ-7500, an IEEE-488 controller/talker/listener, is used to control the IEEE-488 based instruments. This card provides 2 channels for communications via a pair of chips. These INES i9914 chips provide much of the communications functions in hardware, thereby allowing better performance and reliability, and reducing the amount of code in the device drivers.
Three analog to digital converter cards from VME Microsystems International Corporation (VMIC) are used to provide monitoring of the 16 azimuth and 8 elevation drive servos. These cards have 64 channels of 16 bit differential analog input.
The GBT control system depends on the LAN to function. The network has been designed to provide a high level of performance, yet be affordable.
The original design included two networks, one for data transfer, and one for control information. The data load on one Ethernet network would have been too great to allow proper control of the telescope over the same LAN therefore, a data LAN and a M&C LAN were planned. Later, the M&C group decided to use a multi-port RAID to store the backend data, and so the proposed data LAN was unnecessary. At the same time, technology was advancing, bringing to market full-duplex Ethernet switching technology, and 100 Megabit per second Ethernet (Fast Ethernet). Fast Ethernet is a speedy version of standard Ethernet (10 Mbps).