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Computer Gateway Specification and Technical Data
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Introduction
This publication defines the
significant functions of the
Computer Gateway (CG), a
TDC 3000X gateway that provides
an interface from the Local Control
Network (LCN) to a user-selected
host computer. (To connect to a
Digital Equipment Corporation
(DEC) computer, refer to the PLNM
Specification and Technical Data.)
The CG is a node on the LCN, as
shown in Figure 1.
It allows a user-selected host
computer to communicate with:
• All modules on a Local Control
Network,
• Modules on remote Local
Control Networks through
Network Gateways,
• UCN devices, and
• Data Hiway devices.
See System Technical Data for
more information on the CG’s
relationships with other modules
and process-connected devices.
Implementation details are
available in the Computer Gateway
User Manual.
New Computer Gateway Features in R500 LCN Software
• History journal data retrieval
added
• Improved file transfer speed
• Enhanced file transfer
command to list files into a
dataout file (now identical to
Command Processor)
CG03-500
Page 3
Computer Gateway Functions
• Provides a standard interface between the Local Control Network and
the user-selected host computer
• Collects data from other LCN nodes and stores data to other nodes
on request from the computer
• Collects continuous history data from the History Module
• Holds a database that represents computer programs, plus other
computer-prepared data
• Provides for scheduling and/or operator/event demand of computer
programs
• Transmits messages to Universal Station operators
• Receives messages from Advanced Process Manager, Process
Manager, and Multifunction Controller sequences
• Performs file transfer and management functions
• Retrieves LCN History journals to the host computer
DISPATCHER PROGRAM
COMMUNICATION HANDLER
APPLICATION
PROGRAMS
INTERFACE
ROUTINES
SCHEDULER PROGRAM
COMMUNICATION HANDLER
COMMUNICATION HANDLER (LCN)
BISYNCH
OR HDLC
DATA LINK
HOST
PROCESSOR
HOST PROCESSOR
SOFTWARE
ENVIRONMENT
COMPUTER
GATEWAY
CG SOFTWARE
ENVIRONMENT
LOCAL CONTROL NETWORK
DATABASE
ACIDPs & CRDPs
Figure 2 — Host Processor/Computer Gateway/Local Control Network
Relationships 1164
* Bisynch–Binary Synchronous
Communications Protocol Procedure
for Point-to-Point Operation with
Contention, as described in IBM
publication GA27-3004-2, using
transparent Bisynch with EBCDIC
control characters. Maximum block
size is 1024 bytes, and both single
and multiple block transactions
(maximum of 13 blocks) are
supported.
HDLC–Link Access Procedure,
Balanced (LAPB) per C.C.I.T.T.
Document AP VIII-58-E, June, 1984
(Recommendation X.25), and
compliance with FIPS 100
certification.
Functional Description
The relationships of the CG
functions are shown in Figure 2.
The host computer is used for data
collection needed for management
information system or plant
management requirements
(including both current values and
historical data), and a higher level
and wider scope of control than is
possible with any other device.
Typically, CG control strategies are
implemented in conjunction with
Process Manager, Advanced
Process Manager, and Application
Module control strategies.
Host Computer to CG Link
Information is exchanged between
the host computer and the CG over
a serial communication link using
either Bisynch, or HDLC protocol.*
For a Bisynch system, either one or
two links are supported. For an
HDLC system, one full-duplex link
is supported. The standard
interface is an RS422, Type
RS449 interface, with configurable
speed up to 76.8 k baud for a
Bisynch application and 57.6 k
baud for HDLC.
(NOTE: An RS232 interface, with a
configurable speed up to 19.2 K
baud could also be used, instead
of the RS422 interface. However,
the CG is supplied only with the
latter (i.e., RS422) interface as
standard.
Messages between the computer
and CG are managed by a
communications handler program
that queues messages for
CG03-500
Page 4
n depends on transaction type maximum = 512.
Maximum message size (= 504-8)16-bit words.
Transaction
Code
Number of
Words
Number of
Blocks
Block
Number
CG
Identifier
Host Task
Identifier
Message
Tag
Message
Counter
1 2 3 4 5 6 7 8
HEADER BLOCK
MESSAGE BLOCK
9
• • • • • • • • • • • •
n
16-Bit Words
1165
Figure 3 — Message Format
forwarding to the computer, and
distributes messages received
from the host to the appropriate
LCN destination.
Figure 3 shows the message
format. The user is expected to
provide a communications handler
in the computer, along with
computer routines to support CG
functions. No programming of the
CG itself is required; it is
configured in a manner similar to
other LCN nodes through the
Engineer Personality or Universal
Personality of the Universal
Station.
Data Collection and Storage
Computer programs can gain
access to data throughout the
TDC 3000X system. This is
accomplished through seven major
types of CG functions:
• Link Handling/Restart
• Get/Store Point Data
• Get History
• Get/Send Message
• Transfer/Message Files
• Access Journals
• Obtain CG Status
Data Link
The user program must support
either the Bisynch or HDLC link
protocol and link restart
procedures. The remaining
functions below are optional,
depending on the intended
applications.
Data
For Get- and Store-Data calls, the
values are requested either by
single point.parameter names or in
data definition tables that are
defined and built by the user in the
host computer. Sixteen-character
point names are supported. The
tables contain ASCII data point and
parameter names that will be
fetched from and stored to the
TDC 3000X system. The CG
converts the contents of these
tables to internal identifiers that are
used in actual data transfers. Real
values are returned in engineering
unit format. Status of data
collected, as well as status of the
call, is also returned. Up to 300*
parameters can be collected or
stored using a single table.
* Some data types are restricted to
under 300 parameters per
transaction, due to specific
limitations on the maximum message
size on the LCN.
Once built, these tables are stored
in the host computer. Copies can
also reside in the CG for use with
Get-Data calls. The CG also
supports the accessing of Custom
Data Segment arrays using a single
call.
Storing data is handled in a similar
manner to collecting data—either
single point, whole arrays, or data
definition tables can be used.
There are several levels of security
provided to assure proper access
control for writing data to the LCN
and its associated UCNs or Data
Hiways. Writes can be prevented
at the process device, UCN/Hiway,
and program level.
Program security is host-program
dependent. Users may also want
to provide several modes for host
programs to operate, such as test,
restricted, and normal processing.
The CG allows host-programs to
operate in all of the above nodes;
implementation is up to the user.
History
Continuous history data from
History Modules, including fast
history data, can also be obtained
by the computer. This includes not
only PV values, but other real
parameters selected for
historization by the History Module.
Both snapshots and averages can
be accessed. Historical data for up
to 24 parameters can be obtained
in a single call. History can be
CG03-500
Page 5
accessed in relative or absolute
time. Min/max data is also available.
Messages
Computer programs can send
messages to the Universal Station
operator by way of the CG. Such
messages optionally may require
confirmation by the operator. This
allows the computer programs to
request an action and to be
suspended until a confirmation is
received from the operator that the
requested action has been
completed.
Other devices on the LCN can
send character-string messages to
individual application programs in
the computer. These messages
are received by the CG and held,
pending a transfer request by the
application program.
File Transfer and Management
Application programs running in
the host computer can read and
write files on the LCN History
Module. Utilities are provided for
support functions such as file
copy, file re-naming, creation and
deletion of files and directories in
user-created volumes, moving
files from one directory to another,
and listing volume and file
attributes.
History Journal Access
An application program in the host
computer may also retrieve the
history journal events from an
LCN.
History journals include system
journals (i.e., System Status,
System Maintenance, System
Errors and Status Notification
Journals) and process journals
(i.e., Process Alarms, Operator
Messages, Process Changes, and
Sequence-of-Events journals).
Either complete journals can be
retrieved or a specific time interval
may be specified. Specific events
can also be selectively extracted
from the journals, using various
filters.
Gateway Database
Visibility of computer operation at
the Universal Station is provided by
a CG database, which can also be
accessed by using the datacollection
and storage facilities
described above. This database
consists of the following types of
information:
• Advanced Control Interface
Data Points (ACIDPs)
• Calculated Results Data Points
(CRDPs)
Both of these can have optional
custom data segments that hold
data that is associated with
computer programs. For example,
the results calculated by an
optimization program can be stored
in a data point's custom data
segment. The results are then
available to the operator at a
Universal Station, for historization
in the History Module, for
calculations by the Application
Module, etc. In addition, each
ACIDP is associated with a
computer program and represents
that program's status to the LCN.
Each ACIDP also contains security
information about its associated
program, which allows the
program's access to TDC 3000X
data to be restricted; for example,
to read only.
Up to 250 ACIDPs and up to 500
CRDPs can reside in a CG. Each
point can accommodate up to 10
custom data segments, each
segment can contain 250
parameters, and each parameter
can consist of a 1000-element
array (assuming adequate memory
is available).