The PLC Data Server (PDS) makes PLC data available to client programs through a simple API.
The PDS acts as a piece of middleware, simplifying a client's access to a PLC. It takes care of the fieldbus communication to a PLC and abstracts the fieldbus protocol via a tag abstraction - that is, a reference in the PLC is mapped to a tagname and the corresponding data value is mapped to a tagvalue.
The PDS reads in its configuration file and based on the contents, sets up memory structures to hold data for PLC discrete bits, words etc. The configuration file can describe multiple PLCs, and each PLC can have multiple blocks. The communication method for each PLC block is configured as one of a number of supported fieldbus protocols, either over TCP/IP or a serial connection.
The PDS then splits into two distinct processes, one to handle all read requests for PLC data and one to handle all requests to write data to a PLC. In this way the PDS is a concurrent read/write server.
The read process sets up an appropriate fieldbus query for all read/mapped-write blocks found in the configuration file and sits in a continuous loop, running in turn the queries against the PLC. It uses the data returned by the PLC to populate a shared memory segment.
The write process sits polling a message queue and when it receives a request from a client program to write data to the PLC, it constructs the appropriate fieldbus query and sends it to the PLC.
All communication between the PDS and a PLC is via an appropriate fieldbus protocol, either over TCP/IP or a serial connection. All communication between a client program and the PDS is via UNIX Inter-Process Communication (IPC). A client reads PLC data by accessing the PDS's shared memory segment (controlled by semaphores). A client writes data to the PLC by sending a message on a message queue to the PDS. The client/PDS communication is encapsulated in the PDS API.
Optionally, an installation can also enable the PDS network stub. This provides clients with a TCP/IP interface to the PDS, using a simple PDS Network Protocol (PDSNP) encapsulated in the PDSNP API.
To configure, build, and install the PLC Data Server (PDS), type:
./configure
make
make install
This will build the core C interface libraries, the PDS, and a set of utility clients. They will be installed in a tree under /usr/local/pds.
To install to a different installation directory, for example ~/local/pds, type:
./configure --prefix=~/local/pds
make
make install
See ./configure --help for available options.
To build the Tcl/Tk interface libraries and utilities, type:
./configure --with-tcl --with-tclconfig=/usr/lib
make
make install
ensuring the correct path for your tclConfig.sh script.
To build the Python interface libraries and utilities, type:
./configure --with-python --with-includes=/usr/include/python
ensuring the correct path for your python development header files.
N.B.: If both the Tcl and Python bindings require include paths to be specified, then quote them as a single argument to --with-includes, for example:
--with-includes="/usr/include/python3.5m /usr/include/tcl8.6"
In addition, the PDS can be network enabled by building the PDS network stub:
./configure --with-nwstub
make
make install
Caution: The network stub provides an unauthenticated read/write interface to the PDS. By default, the PDS network stub will listen on localhost, port 9574, so access to the PDS host is still required by clients. However, in this case, you may as well just use the PDS without the network stub.
To configure the PDS network stub to listen on all network interfaces, and on a different port (9999, say), type:
./configure --with-nwstub --with-nwstub-host=0.0.0.0 --with-nwstub-port=9999
make
make install
Enabling the PDS network stub with the --with-nwstub option, will configure the PDS control script (pds) to start/stop the PDS network stub as well as the PDS.
To run the PDS, cd to the bin directory (under install directory) and run the PDS control script.
To start the PDS type pds start, to stop the PDS type pds stop. An example PLC configuration file has been provided as a template for your own configuration and is located in the data directory.
By default, the PLC configuration file is called plc.cnf. It is a plain-text formatted configuration file.
To view the data from your configured PLCs, run the utility program plcmm. See also the other utility programs, installed under the PDS bin directory.
# Project: PDS Demo
# Purpose: PLC Communications Configuration File
# Author: Paul M. Breen
# Date: 2000-09-27
#
# Format: /protocol/function/path/IP_address:port/base_address/poll_rate
# OR /protocol/function/path/"tty_device"/base_address/poll_rate
# <white space>tagname tagreference wordlength
###############################################################################
# PLC 1
###############################################################################
# Communicate via ModBus/TCP/IP to the PLC identified as IP address 10.4.8.190,
# TCP port 502, Unit ID 0. Read 2 bits (coils) starting at reference 00101.
/MB_TCPIP/BREAD/0/10.4.8.190:502/00000/10000
pds_mb_read_bit1 00101 0
pds_mb_read_bit2 00102 0
# Communicate via ModBus/TCP/IP to the PLC identified as IP address 10.4.8.190,
# TCP port 502, Unit ID 0. Write upto 2 bits (coils) starting at reference
# 00201.
/MB_TCPIP/BWRITE/0/10.4.8.190:502/00000/10000
pds_mb_write_bit1 00201 0
pds_mb_write_bit2 00202 0
# Communicate via ModBus/TCP/IP to the PLC identified as IP address 10.4.8.190,
# TCP port 502, Unit ID 0. Read 2 16 bit words (registers) starting at
# reference 40101.
/MB_TCPIP/WREAD/0/10.4.8.190:502/40000/10000
pds_mb_read_word1 40101 16
pds_mb_read_word2 40102 16
# Communicate via ModBus/TCP/IP to the PLC identified as IP address 10.4.8.190,
# TCP port 502, Unit ID 0. Write upto 2 16 bit words (registers) starting at
# reference 40201.
/MB_TCPIP/WWRITE/0/10.4.8.190:502/40000/10000
pds_mb_write_word1 40201 16
pds_mb_write_word2 40202 16
###############################################################################
# PLC 2
###############################################################################
# Communicate via ControlNet/TCP/IP to the PLC identified as IP address
# 10.4.8.191, TCP port 44818, Routing Path 1.0 (backplane, slot 0). Read 1
# bit identified as logical_1.
/CIP_TCPIP/BREAD/1.0/10.4.8.191:44818/logical_1/10000
pds_cip_read_bit0 0 0
# Communicate via ControlNet/TCP/IP to the PLC identified as IP address
# 10.4.8.191, TCP port 44818, Routing Path 1.0 (backplane, slot 0). Read 1
# bit identified as logical_2.
/CIP_TCPIP/BREAD/1.0/10.4.8.191:44818/logical_2/10000
pds_cip_read_bit1 0 0
# Communicate via ControlNet/TCP/IP to the PLC identified as IP address
# 10.4.8.191, TCP port 44818, Routing Path 1.0 (backplane, slot 0). Read 4
# 16 bit words from the Input_Data_Int[] array
/CIP_TCPIP/WREAD/1.0/10.4.8.191:44818/Input_Data_Int[]/10000
pds_cip_read_word0 0 16
pds_cip_read_word1 1 16
pds_cip_read_word5 5 16
pds_cip_read_word7 7 16
# Communicate via ControlNet/TCP/IP to the PLC identified as IP address
# 10.4.8.191, TCP port 44818, Routing Path 1.0 (backplane, slot 0). Write upto
# 4 16 bit words to the Input_Data_Int[] array
/CIP_TCPIP/WWRITE/1.0/10.4.8.191:44818/Input_Data_Int[]/10000
pds_cip_write_word0 0 16
pds_cip_write_word1 1 16
pds_cip_write_word2 2 16
pds_cip_write_word3 3 16
# Communicate via ControlNet/TCP/IP to the PLC identified as IP address
# 10.4.8.191, TCP port 44818, Routing Path 1.0 (backplane, slot 0). Read from
# the 16 bit word identified as CIP tag parts
/CIP_TCPIP/WREAD/1.0/10.4.8.191:44818/parts/10000
pds_cip_read_parts 0 16
# Communicate via ControlNet/TCP/IP to the PLC identified as IP address
# 10.4.8.191, TCP port 44818, Routing Path 1.0 (backplane, slot 0). Write to
# the 16 bit word identified as CIP tag parts
/CIP_TCPIP/WWRITE/1.0/10.4.8.191:44818/parts/10000
pds_cip_write_parts 0 16
# Communicate via ControlNet/TCP/IP to the PLC identified as IP address
# 10.4.8.191, TCP port 44818, Routing Path 1.0 (backplane, slot 0). Read 3
# 32 bit words (hi-word, lo-word) from the input_dint[] array
/CIP_TCPIP/LREAD/1.0/10.4.8.191:44818/input_dint[]/10000
pds_cip_read_hi_dint0 0 16
pds_cip_read_lo_dint0 0 16
pds_cip_read_hi_dint1 1 16
pds_cip_read_lo_dint1 1 16
pds_cip_read_hi_dint2 2 16
pds_cip_read_lo_dint2 2 16
# Communicate via ControlNet/TCP/IP to the PLC identified as IP address
# 10.4.8.191, TCP port 44818, Routing Path 1.0 (backplane, slot 0). Write upto
# 3 32 bit words (hi-word, lo-word) to the input_dint[] array
/CIP_TCPIP/LWRITE/1.0/10.4.8.191:44818/input_dint[]/10000
pds_cip_write_hi_dint0 0 16
pds_cip_write_lo_dint0 0 16
pds_cip_write_hi_dint1 1 16
pds_cip_write_lo_dint1 1 16
pds_cip_write_hi_dint2 2 16
pds_cip_write_lo_dint2 2 16
###############################################################################
# PLC 3
###############################################################################
# Communicate via DataHighway (DF1)/serial/TCP/IP to the PLC identified as
# IP address 10.4.8.192, TCP port 502, station ID 6. Read 6 16 bit words
# starting at the $N10:0 file
/DH_SERIAL_TCPIP/WREAD/6/10.4.8.192:502/$N10:0/10000
pds_dh_read_word0 0 16
pds_dh_read_word1 1 16
pds_dh_read_word2 2 16
pds_dh_read_word3 3 16
pds_dh_read_word5 5 16
pds_dh_read_word17 17 16