A DISTRIBUTED AND COLLABORATIVE PLC LAB FOR THE SPALLATION NEUTRON
SOURCE\*

J. Y. Tang, J. D. Smith, Brookhaven National Laboratory, Upton, NY, USA

W. R. DeVan, R. E. Battle, Oak Ridge National Laboratory, Oak Ridge, TN,
USA

S. A. Lewis, C. A. Lionberger, Lawrence Berkeley National Laboratory,
Berkeley, CA, USA

D. P. Gurd, Los Alamos National Laboratory, Los Alamos, NM, USA

Abstract

The control system for the Spallation Neutron Source (SNS) will be
developed collaboratively at five different national laboratories. It is
expected that many PLCs will be used. Several of the collaborating labs
have experience with PLC systems, but not all use the same hardware and
some of the hardware is obsolete. New PLC technology is now available. A
collaborative and distributed PLC development lab has been established
to help select the most suitable industrial systems and devices for the
SNS project. In this paper, we present the configurations, features,
performance, reliability and a cost comparison of the prototype systems
we have reviewed. The selected PLC development projects cover the
following areas: using soft PLCs; linking PLCs with TCP/IP-based
protocols or the ControlNet™ network; integrating input/output modules
via the DeviceNet™ network; using PLC I/O modules such as Flex-I/O™ and
G3™ without ladder-logic to exploit their form-factor and electrical
properties; and comparing the low-cost PLC systems against the most
popular brands. The integration of the PLC prototypes with EPICS will be
discussed.

1  Introduction

The Spallation Neutron Source (SNS) is an accelerator-based 1 -- 2 MW
pulsed neutron source to be built in Oak Ridge, Tennessee. A fully
integrated control system for the SNS facility will be developed
collaboratively at five different national laboratories and the
integration of the control system will be based upon the widely used
EPICS control system toolkit \[1\]. It is expected that many PLCs will
be used in the system. Several of the collaborating labs have experience
with PLC systems, but not all use the same hardware. Some of the
hardware models have been in the market for more than fifteen years
while new PLC technology is now available. A collaborative and
distributed PLC development lab has been established to help select the
most suitable industrial systems and devices for the SNS project.

In this paper, we first describe the configurations of the prototype
systems we have reviewed. Then the following topics will be covered: 1)
the two methods that we have chosen to link PLCs with TCP/IP based
protocols; 2) the comparison of Allen-Bradley's PLC-5, SLC500, and
ControlLogix PLCs on costs, performance, functionality, and reliability;
3) the experience of using Group3™ technology; 4) the experience of
using Flex-I/O™ technology; 5) integrating I/O modules via DeviceNet™.
Finally, the recommendations on selecting PLCs for the SNS project will
be given.

2.  The SNS PLC lab

2.1 The configuration of the SNS PLC lab

To help select most suitable industrial systems and devices for the SNS
project, a collaborative and distributed PLC development lab has been
established as shown in Figure 1.

![](media/image1.png){width="3.3958333333333335in"
height="3.122916666666667in"}

Figure 1: The SNS PLC Lab

The SNS PLC lab is configured collaboratively at Argonne National
Laboratory (ANL), Brookhaven National Laboratory (BNL), Lawrence
Berkeley National Laboratory (LBNL), Las Alamos National Laboratory
(LANL), and Oak Ridge National Laboratory (ORNL). Each collaborating lab
has been working on one or two areas of the selected PLC development
projects:

-   At ANL, it has shown the success of using EPICS as a soft PLC that
    communicates with Allen-Bradley I/O modules via a VME Remote I/O
    scanner. The I/O control logic is implemented in EPICS databases.
    For details, see \[2\].

-   At BNL, the prototype systems of linking PLCs with TCP/IP based
    protocols and integrating I/O modules via DeviceNet are under
    development.

-   At LBNL, both Group3 and Flex-I/O technologies have been used for
    their prototype systems.

-   At LANL, a PLC EPICS record is under development to streamline PLC
    to EPICS (This topic will not be covered).

-   At ORNL, DeviceNet™ is being used for an optional link between VME
    EPICS controller and PLCs.

2.1  Linking PLCs with TCP/IP based protocols

Ethernet technology has advanced rapidly in recent years. The Control
and Information (CIP) protocol for real-time on Ethernet is being
developed by The ControlNet™ International, Ltd. Two methods of linking
PLCs with the SNS control system via Ethernet have been investigated:

-   *Via a PLC Information Server*

> A PLC Information Server is configured with a portable Channel Access
> server and the RSLinx™ (PLC Ethernet interface driver) running on a
> NT™ station This method centralizes the communication between PLCs and
> the upper layer of the control system. It avoids redundant PLC I/O
> requests to PLCs. A PLC processor, therefore, is able to dedicate its
> time for more time critical low-level tasks.

-   *Directly communicate with the Ethernet interfaced PLCs*

> Another approach is to implement an EPICS PLC Ethernet driver that
> directly communicates with PLCs. However, the Ethernet protocol for
> Allen-Bradley programmable controllers is Allen-Bradley proprietary
> information. A Confidentiality Agreement for the protocol between
> Rockwell Automation and Brookhaven National Laboratory has been
> signed. An EPICS driver for Allen-Bradley Ethernet PLC is under
> development for evaluation purposes only.

2.  PLC-5, SLC500, or ControlLogix

Allen-Bradley PLC-5 and SLC500 are widely used industrial control
processors. PLC-5 has been in the market more than fifteen years and
SLC500 for more than ten years. Both processors are currently used for
other projects at all five SNS collaborating labs. There are many
engineers at the SNS collaborating labs who have many years of
experience with PLC-5 and SLC500. On the other hand, a new PLC model,
ControlLogix, has been in the market only for about two years.
ControlLogix is about 8\~10 times faster and 30% cheaper than PLC-5. The
comparison of PLC-5, SLC500, and ControlLogix on costs, performances,
functionality, and reliability is summarized in Table 1.

Table 1: PLC-5, SLC500, or ControlLogix

+---------------------+-----------+--------------+--------------------+
|                     | **PLC-5** | **SLC500**   | **ControlLogix**   |
+---------------------+-----------+--------------+--------------------+
| **Costs**           | 1         | 35%          | 65%                |
+---------------------+-----------+--------------+--------------------+
| **Years in Market** | 15        | 10           | 2                  |
+---------------------+-----------+--------------+--------------------+
| #### Reliability    | Similar   |              | Too new to be      |
|                     | MTF       |              | measured           |
|                     | records   |              |                    |
+---------------------+-----------+--------------+--------------------+
| #### Functionality  | Most      | 2nd          | Least (it has the  |
|                     |           |              | capability to talk |
| (Special I/O        |           |              | with PLC-5 I/O     |
| modules             |           |              | modules via        |
| availability)       |           |              | adapter)           |
+---------------------+-----------+--------------+--------------------+
| #### Performance    | Similar   |              | 8 \~ 10 times      |
|                     | per       |              | faster than PLC-5  |
|                     | formances |              |                    |
+---------------------+-----------+--------------+--------------------+
| **In-house          | About 40  |              | Only one recently  |
| expertise           | engineers |              | has been trained   |
| availability**      | at BNL    |              |                    |
+---------------------+-----------+--------------+--------------------+

2.3 Experience using Group3™ Technology

The Group3™ interface combines fiber-optic cable, a VMEbus loop
controller, and up to 32 slave units. LBNL intends the fiber to provide
up to 65kV standoff and significant protection against sparking and
ground loops in the front-end. The slave units are of two varieties: a
mini-chassis holding up to three boards of mixed type; or a combination
unit (CNA) with analog/digital in/out.

LBNL has written EPICS driver support for the Group3 VME loop
controller, as well as specific EPICS device support for a variety of
slave units (CNA, analog, digital, stepper motor, DC motor, serial).
Additionally, the remote diagnostic/ configuration capability (running
concurrently with normal I/O over the fiber loop) has been extended to
work via Channel Access, and a client application is provided to emulate
the hand-held unit that connects directly to the diagnostic port. A
utility has been written to save, restore, and compare the contents of
configuration RAM for any slave unit.

With the CNA unit used one-to-one with each power supply, we exploit one
of the relays to loop the DAC back (under program control) into the
second ADC channel in place of the external readouts. The motor units
are matched one-to-one with the mechanism being controlled, since they
include digital inputs for limit switches and analog inputs for "follow
pots."

2.4 Experience using Flex-I/O™ Technology

Flex-I/O™ is an Allen-Bradley interface family usable with any of their
PLC units or directly with their VMEbus scanner. LBNL has is using
Flex-I/O™ in both configurations for its convenient form-factor
(modular, DIN-rail), low cost, and rugged electrical characteristics.
For both interlock and vacuum systems in the front-end, we will combine
the PLC/5 ladder-logic with Flex-I/O "stacks" situated close to clusters
of pumps, valves, and gauges. For extensive temperature and monitoring,
we will connect it directly (no ladder-logic).

LBNL has added EPICS device and record support to enable use of the
Flex-I/O 1794 units for digital in/out (-IB16 and -OW8) and analog
in/out (-IE8, -OE4, -IE4XOE2). Additional support has been added to
enable block transfer operations to a PLC/5 configured simultaneously in
adapter-mode (seen by VMEbus scanner) and in scanner-mode (controlling
Flex-I/O).

2.5 Integrating I/O modules via DeviceNet™

DeviceNet™ is an open network standard. There are more than 150
companies that develop products that are compatible with the DeviceNet
network. It is a simple, networking solution that reduces the cost and
time to wire and install industrial automation devices, while providing
interchangeability of devices from multiple vendors.

Two VME DeviceNet™ interface modules are currently under evaluation at
BNL and ORNL. The EPICS DeviceNet™ drivers for these two modules are
currently under the development by the joint effort from the two labs.
In Table 2, EPICS links, DeviceNet™ features and costs of these two
modules are listed.

Table2: IP-DeviceNet vs 5136-DN-VME

+-----------------+-----------------------+----------------------------+
|                 | **IP-DeviceNet**      | **5136-DN-VME**            |
+-----------------+-----------------------+----------------------------+
| **EPICS link**  | The IP module is      | An EPICS driver for this   |
|                 | configured with       | VME DeviceNet™ scanner     |
|                 | VIPC616 VME IP        | will be developed by ORNL  |
|                 | carrier. The EPICS    | and BNL                    |
|                 | driver is under the   |                            |
|                 | development at BNL    |                            |
+-----------------+-----------------------+----------------------------+
| **DeviceNet     | I/O Slave Messaging:  | Master scanner Messaging:  |
| features**      | polling               | Explicit Peer-to-Peer      |
|                 |                       |                            |
|                 |                       | I/O Slave Messaging: Bit   |
|                 |                       | Strobe/Polling/Cyclic/COS  |
+-----------------+-----------------------+----------------------------+
| **Cost**        | IP-DeviceNet: \$475   | \$1590                     |
|                 |                       |                            |
|                 | IP-Carrier (carry 4   |                            |
|                 | modules) : \$1020     |                            |
+-----------------+-----------------------+----------------------------+

3.  conclusions

It is desirable to standardize on a limited number of PLC manufactures
and models to reduce effort to develop EPICS software drivers, to reduce
operations effort to support multiple PLC models, to obtain the best
pricing by pooling of orders and to concentrate efforts to develop best
practices. However, there is always a trade-off when one chooses one
model over the other. A collaborative PLC lab will help make such
decisions. We recommend the following:

-   Consider the availability of in-house expertise when calculating the
    costs;

-   Choose as new technology as possible for easier future
    upgradability;

-   Use open standard networks for interchangeability of devices from
    multiple vendors.

REFERENCES

\[1\] B. DEVAN and etc., "Plans For A Collaboratively Developed
Distributed Control System For The Spallation Neutron Source", PAC99,
New York

\[2\] M. KRAIMER and etc., "EPICS: Allen Bradley Driver and Device
Support", http://www.aps.anl.gov/xfd/SoftDist/swASD/allenBradley