The trend 

towards industrial PC's has received a permanent boost from the development of slot PC/ PLC boards. The host CPU takes over the visualization function, while the slot PC performs the control tasks within a system.

In the past, the industrial PC has often been used for visualization together with a conventional PLC control. The connection between the PLC and visualization system is usually effected via a serial or a network interface. This always leads to a bottle-neck, especially as communication has usually been performed via a DDE connection.

Slot PLC boards such as the "SPC" by TRS Fieldbus Systems are becoming increasingly popular. The core of the board is a self-contained PC. This sub-PC is decoupled from the ISA bus by a dual-ported RAM. The sub-PC is then turned into a slot PLC by the firmware of the PLC interpreter.

The structure of the process image in the dual-ported RAM is independent of the field bus. So is the programming, which is effected on an external laptop via an RS-232 interface. Only the data module containing the I/O definition of the field bus depends on the field bus.

The slot PLC is programmed via an AS-511 protocol, thus allowing various common programming interfaces to be used. There is a clear separation between the visualization system and the PLC. The data transfer between both program parts is effected via the dual-ported RAM, which is logically much faster than the conventional serial connection.

The visualization system is programmed by Visual Basic or Delphi, for example, or via any other visualization package available on the market. For communication with the dual-ported RAM of the slot PLC, the programmer can use DLL functions and DDE servers.

This makes the system much easier than the soft PLC. The screen of the host PC is used for visualization and the screen of the laptop as the PLC programming interface.

The advantage of the "PLC" by TRS Fieldbus Systems becomes even more apparent. When using further slot PLC boards on the same passive ISA bus. Communication between the slot PLC boards is ensured either by mailboxes within each dual-ported RAM, whose data transfer is effected by the host CPU, or by an additional CAN bus link between all the slot PLC boards involved.

TRS_Grph.JPG (28197 bytes)

  • SPC assembly serving as a PLC, for integration in any PC

    With field bus interfaces, e.g. for

    - DeviceNet SPC-520
    - Lightbus SPC-200
    - Profibus-DP SPC-300
    - INTERBUS-S SPC-400
    - CAN bus SPC-5XX


  • The "SPC" CPU can be extended by a PC-104-VGA board, hard disk, floppy drive and external keyboard, thus providing you with a "PC within a PC"

  • PC kernel with 586/133 MHz and Pentium 200

  • The PLC is programmed via the serial port COM 1 of the "SPC".  This means that the PLC can be programmed not only with the Host PC, but also via an external laptop or a Siemens®  PU.  As a protocol, AS511 (or alternatively an IBH Softec protocol is used at the serial port.

  • The PLC has the performance range of a SIEMENS 945 CPU®  with respect to operations set and speed.

  • The PLC communicates with the higher-level host PC via a dual-ported RAM that provides the process image.

  • Programming in S5 / S7® (STL, SFC, LAD, graphic sequences) or in IEC1131-3

  • CAN bus link for "SPC" - "SPC" communication.

  • DDL and DDE interfaces with the host CPU (e.g. visualization)

  • Besides a lightbus connection, there are further functions integrated on the motherboard:

    • interface with UPS power supply unit (data remanence)

    • parallel interface for special keys and LEDs

    • status display (7-segment display)

    • switch for start/stop, normal/special operation

    • external power supply (12 V... 24 V DC)

    • 128 Kbyte buffered SRAM

    • 4 Mbyte (10 Mbyte) flash

    ®registered trademark of Siemens AG

visualization of the PLC program is usually effected with an external laptop or programming unit via a serial port. By this means, online interventions in the normal way are possible. While the PLC is viewed as it were from the outside via the serial port, the host CPU views the PLC from the inside via the dualported RAM. An action in the host PC, e.g. pressing a key of the visualization interface, triggers a reaction in the PLC which can then be observed simultaneously on the programming unit. The dual-ported RAM interface between host PC (visualization) and SPC (PLC) connects both program parts without either part directly influencing the other. Screen masks, higher-level routines etc. have no effect on the cycle time of the PLC, as these run completely autonomously on self-contained hardware.

In each cycle, the slot PLC refreshes the process image within the dual-ported RAM that constitutes the interface between PLC and host PC, so that the user is supplies with all the data of the PLC. For visualization of the process image of the SPC (slot PLC), all modern tools can be used. DLLs (Dynamic Link Library) as well as various DDE interfaces (Dynamic Data Exchange) are available from TRS, for example. Adaptions to various commercially available visualization packages have been and are being created. We are open to any requests of the nature.

If several slot PLC boards are combined on a common ISA backplane, the host PC can of course write and read data from all PLC boards, i.e. visualize several PLCs simultaneously. Available programming interfaces are S5 for Windows and IEC1131. The picture above shows some extracts from the PLC programming interface.


back | Home

Copyright © 2000 TRS Fieldbus Systems, Inc. All rights reserved.
Revised: July 14, 2000 .