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JUL 2~ 5~ DEVICE CONTROL AT CEBAF* C -
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S. Schaffner, B e , V. Bookwalter, B. Bowling, K. Brown, L. Doolittle, T. Fox, S. Higgins, A. Hofler,
G. Lahti, P. Letta, B. Montjar, N. Patavalis, J. Tang, W. Watson, C. West, D. Wetherholt, K. White, S.
Witherspoon, and M. Wise, Continuous Electron Beam Accelerator Facility, Newport News, VA, USA
CEBAF has undergone a major conversion of its acceler-
ator control system from TACL to EPICS, affecting device c
control for the RF system, magnets, the machine protection t
system, the vacuum and valves, and the diagnostic systems in- 1
cluding beam position monitors (BPMs), harps, and the cam-
era and solenoid devices (beam viewers, faraday cups, optical A
transition radiation viewers, synchrotron radiation monitor, c
etc.). Altogether these devices require approximately 125,000 I
EPICS database records. The majority of these devices are e
controlled through CAMAC; some use embedded micropro- T
cessors (RF and magnets), and newer interfaces are in VME. e~
The standard EPICS toolkit was extended to include a driver v
for CAMAC which supports dual processors on one serial E
highway, custom database records for magnets and BPMs, b
and custom data acquisition tasks for the BPMs.
I. Differences Between TACL and EPICS .r
The systematic differences between TACL and EPICS th
have been well-documented, see [I & 2]. From the standpoint th
of the low-level application developers at CEBAF involved in th
the conversion of the control system, a few key differences in
stand out. d
A. Hardware Control in
In TACL, control algorithms were stored as elements in a c
logic grid where different subsystems were distinguished by
location in the grid. This inhibited independent development p
because only one subsystem at a time could access the grid to si
work on its control algorithm. In EPICS, the control algo- o
rithms are stored in independent databases which are not com- E
bined until the system is loaded onto the input-output T
Another difference is the frequency with which logic ele- I
ments or database records are processed. Because TACL ran
on UNIX machines, processing frequency was non-determin- a
istic. EPICS runs on processors which use a real-time kernel, s
therefore the rate at which EPICS database records are pro- it
cessed is deterministic.
In TACL the logic grid was processed sequentially from
left-to-right and cycled at a rate determined by the size of the
logic grid and the speed of the processor. EPICS has a greater C
variety of execution options including differential scan rates,
passive processing (records process only when triggered by s
another record), and software and hardware event-triggered te
processing. EPICS also provides a tool which makes it easy to tr
set up finite state machines (sequencers) which give a finer de- c
gree of control over how database records are processed. S
'Supported by U.S. DOE contract DE-AC05-84ER40150. p
In TACL, predefined defaults could be set for signals
coming from a remote computer when communication be-
ween two computers was lost. In EPICS, signals retain the
ast known value before communication was lost.
TACL provided predefined logical operators like OR,
AND, NAND, and inverter as well mathematical operators in-
luding transcendental functions. TACL also provided digital
ogic elements such as Flip Flops, multiplexers/demultiplex-
rs, comparators, and words-to-bits, bits-to-words convertors.
he number of inputs and outputs for these elements could be
easily set by the application developer. EPICS does not pro-
ide these operators in a pre-defined manner. It is not easy in
PICS to expand the number of inputs and outputs to a data-
The operator interface (OPI) portion of the control system
EPICS is fully integrated into the XWindows system while
he TACL OPI was built using a proprietary graphics system
hat ran only on HP workstations. One impact is that in TACL,
he OPI was limited to a single window per workstation while
EPICS the OPI can display multiple windows. In TACL,
display screens tended to contain a lot of information and be-
ame very crowded. In EPICS, it is possible to modularize the
formation presented to operators which makes it easier to fo-
us on a specific task.
The EPICS OPI does not have the same concept of a
ush-button as did TACL. In TACL it was possible to tie two
gnals to a single push-button so that the operator could set
ne signal and read back the results on a separate signal. In
PICS, push-buttons are tied to a single signal only. Also, the
ACL OPI had a predefined symbol that interacted with the
'ord-to-bits and bits-to-words logic element which would al-
ow an operator to set and/or monitor individual bits. A pair of
ord-to-bits and bits-to-words records were added to EPICS
CEBAF early in the conversion process but a matching
symbol was not added to the OPI until much later. Even now,
is not easy to set individual bits from the OPI in EPICS.
II. Conversion From TACL to EPICS
. RF Prototype and CAMAC Driver
The first system to be converted was the RF system which
applies the power needed to accelerate the beam. The RF sys-
m is distributed across 350 modules and each module is con-
olled by a microprocessor. The microprocessors
communicate to the control system via CAMAC buffer cards.
ome additional control and monitoring functions are provid-
d by CAMAC cards which are not integrated into the micro-
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Schaffner, S.; Barker, D. & Bookwalter, V. Device control at CEBAF, article, August 1, 1996; United States. (digital.library.unt.edu/ark:/67531/metadc664766/m1/1/: accessed June 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.