|
"Traditional" PLC I/O
This category of I/O products are rack mounted, modular I/O systems that provide industrial
packaging, reasonable cost and performance for most applications, and easy troubleshooting. The
I/O racks and modules are normally mounted in a cabinet and signal wires are run to the cabinet
from the sensors.
Tealware: SoftPLC Corporation provides this I/O system with their brand-name Tealware™ I/O.
This system provides high-speed local I/O up to 1024 digital points (or up to 32 total modules) using a Tealware CPU.
The I/O scan for most local systems is 1msec. A remote I/O network which runs at 2.5MBaud
allows for up to 16K digital points (64 racks = 512 modules) at a distance of up to 20,000 feet from
the CPU. The remote I/O network allows for 4 local racks per remote drop at local I/O speeds,
with about a 2msec delay per drop (not per rack), for up to 15 remote drops. Local and Remote
Tealware I/O can be used with the SoftPLC In Tealware CPU modules and an interface card that
can be installed in any PCI slot of a PC (Cat. No. SPO-TWRIO). Tealware is a cost effective, fast
I/O system suitable for most industrial applications.
A-B RIO: SoftPLC supports Allen-Bradley Remote I/O (RIO) networks (1771, 1747, Block, Flex,
Point, and other compatible I/O systems and devices) via the A-B 1784-PKTS/PKTX/PKTXD
interface cards that can be installed in an ISA slot of a PC, such as a SoftPLC Processor™. Up to 8
-PKTx cards are supported in a single SoftPLC system, for up to 16 RIO networks. A-B does not
allow third parties to support their local I/O. The network speed is 115.2KBaud maximum, and
throughput on the A-B RIO network is the same as that of an A-B PLC-5 or SLC-500 system,
including the 7-8msec delay per rack caused by the Remote I/O Adapter (eg: 1771-ASB). If Block
Transfer is used for analog or other intelligent I/O modules, the same restrictions of one Block
Transfer per adapter per scan apply. A-B publishes formulas that can be used to determine the I/O
Throughput Time in a RIO system. By using multiple RIO channels, users can improve the
Throughput, by setting scan sequences and offloading intelligent modules to 1/rack. SoftPLC, in its
I/O Scan simply reads/writes data from its RAM data table over the backplane to the RAM on the
KTx card(s), therefore, the I/O Scan is normally less than 1 msec. Unless the I/O is already in
place as in an upgrade situation, A-B RIO is not normally a recommended solution due to high cost and slow Throughput
Time, along with cumbersome programming and delays associated with Block Transfer.
Ethernet I/O
Ethernet I/O is becoming very popular, primarily due to the low cost of interfacing, cabling, switches,
fiber optic equipment, and wireless support. All ethernet I/O is not compatible, the protocol layer varies
by vendor. A number of protocols have been adopted by industrial equipment vendors, and many field bus protocols are also
available over ethernet.
SoftPLC supports Modbus IP for ethernet I/O. This is the first available ethernet I/O protocol, and has broad support by
vendors throughout the world. SoftPLC provides compatible hardware in their Tealware
and A-series I/O products.
Modbus IP comes in TCP and UDP "flavors." Due to the size of the TCP/IP stack, ethernet I/O performance is typically 2-10msec per drop,
although we have seen some implementations at 30-50msec per module.
Field Bus I/O
This category of I/O products consists of standardized protocols for distributed I/O networks.
Many vendors provide sensors and I/O modules for these networks. Field Bus I/O provides a way
to distribute the I/O along a machine/process to minimize wiring runs back to the control cabinet,
as well as providing intelligence in sensors for better diagnostic information or localized control at
the sensor level. A number of papers, pamphlets and comparison charts are available from the
product and interface card vendor on the advantages, disadvantages, and differences between these
networks, therefore, this information is not provided here.
SoftPLC supports Field Bus networks through interface cards manufactured by Hilscher, GmbH
. PCI, ISA, and PC/104
form factors are available for a wide variety of networks including DeviceNet, Profibus, Interbus, CANopen, SERCOS and many others.
Computer I/O
This category of I/O products are I/O boards that install directly on the PC backplane and are wired directly or interfaced to I/O
blocks, such as Opto modules installed on a terminal board. Normally, the I/O board is connected to the
terminal board(s) via ribbon cables. The advantages of this category I/O are typically lower cost with very
high speed and/or high performance specifications. The disadvantage is that with higher I/O count
requirements, the system becomes difficult to troubleshoot and cabinet space intensive. Also, extreme care
must be taken in these installations to ensure proper isolation from external noise from entering into the
SoftPLC through the I/O system, as many of these boards and modules do not provide isolation such as
typically found in PLC I/O or other industrial I/O products. I/O boards are available for ISA, PC/104, PCI,
VME and other PC backplanes.
SoftPLC has written a number of drivers for different vendor I/O products on a number of different PC
backplane types, as have other third parties. A sample driver is included with the SoftPLC C Programmer's
Toolkit. Most C/C++ programmers have found that a driver of this type can be written and tested in 1-3
days time using the sample as a starting point.
Since the I/O board is on the PC backplane, the I/O Scan is normally much less than 1 msec. The I/O
Throughput is based on the hardware selection.
Other computer I/O interfaces through the PC serial port or parallel port. Typically, these I/O systems are
slow due to baud rate limitations as well as the synchronous nature of the I/O driver (each module must be
communicated with individually one at a time). SoftPLC provides one of this type of driver for the
Advantech ADAM 4000 series and clones.
|
