GEM80 PLC Controllers

Front of GEM80 500 Controller

Article on GEM80 Systems


GEM80 Controllers

Firstly the development of the GEM80 range of programmable logic controllers began in the late 1970’s. It became the PLC of choice for major manufacturing industries. Consequently GEM80’s are found in a diverse range of industries such as Metals, Automotive, Water, Mining, Power Generation, Food, and Oil and Gas.

Then under the GEC Industrial Controls banner, the uptake of GEM80 PLC’s in the Mid 1980’s was extensive, because of this they boasted approximately a quarter of the UK’s PLC market.

Since early GEM 80 controllers operate in the same way as the current models, all allbeit in a few specific areas, GEC/CEGELEC/ALSTOM have a general policy of forward compatibility. Consequently the newer GEM80 controllers will need very few modifications. Allowing older systems to be replaced by the current 400 and 500 series systems

Controller architecture and operation

Lets consider the basic components for all GEM 80 controllers are microprocessor modules, memory modules and a power supply unit. These components are connected onto the central highway, to allow data communication between them.

The central highway consists of a printed circuit board (PCB), containing a number of edge connectors into which the GEM 80 controller modules are fitted. This PCB forms a “backplane” through which memory and processor modules can interchange data.

GEM80 System components

GEM Processors

The GEM80 400/500 controllers contain main microprocessor module. 

The I/O highway ribbon that connects the GEM 80 I/O subrack to the controller is connected to this processor. It is responsible for accepting the input data from the input modules and storing the data in an assigned memory area.

Once the program has been executed by the new output states will be calculated and placed in the memory area assigned to output data. The Processor then moves the output data from memory, transferring it to the output modules in the GEM 80 I/O subrack.

The Processor also handles serial communication data transfers from the two standard GEM 80 serial ports. The processor transfers data into the serial link assigned memory area as well as transferring data from memory out via the serial ports. As well as controlling the operation of the standard serial links, the serial communication link to the GEM 80 programmer, connected to the programming port.

Ladder diagram execution

The Processor’s task within the GEM 80 controller is to execute the user program. To do this it requires the state of the input data tables, these are obtained from the memory having been placed there by the I/O Processor. With this information the processor is able to execute the program.

As a result of program execution the updated states of the outputs are generated. These are then loaded into the memory. Finally the  Processor then retrieves and transfers the data to the output modules.

Memory Modules

The GEM400 can contain memory modules provide a large storage area, that is divided into smaller sections. These are then used for their assigned tasks. The different sections of information that may be held within GEM 80 controllers memory include the following, program instructions, data tables, video memory and mailboxes, used for inter-module message passing

GEM80 Programmers

System programmer and emulator

All GEM80 systems can be programmed using many different programming devices:

  • Firstly the original GEC Portable programmer circa 1980, a dedicated black box programmer. Required a separate cassette deck and din plug to save and load programs.
  • The GEM80 System Programmer a common site in the mid 1980’s with it’s black front and yellow outer casing. Included a digital tape drive for loading and saving programs. Later models were sprayed in CEGELEC’s grey colour. Enhanced programmers also included a floppy disk drive for program storage, although the formatting is not PC compatible.
  • In 1986 a third party company Advanced Technical Software Ltd produced the System Programmer Emulator the first PC DOS based GEM80 programmer. This more or less copied the system programmer menu screens, apart from the copy compare option. This is due to the fact that the PC had a disk drive rather than the digital tape drive. This product was sold by GEC at Kidsgrove under licence from ATS.


  • Mid 1980’s GEC projects at Rugby developed their own DOS based programmer GPP (GEC Projects Programmer). This was used primarily on GEC projects jobs and was therefore not sold as a product, only as part of a GEC project. The format is much the same as the ATS system programmer emulator, the system being a basic system programmer emulator package. 
  • In 1990 the renamed CEGELEC industrial Controls produced a Windows GEM80 programmer called GEMESYS 3 that ran under Windows 3.1 (and now Windows 95,95 etc.). This product had a very slow uptake with most customer’s preferring to stick with the DOS Based ATS programmer which had now been overhauled and named the Universal GEM Programmer.
  • Advanced Technical Software The company updated the basic emulator and developed extra menus, screens and search options to provide the quickest means of programming and monitoring a GEM80 system. Although still DOS based if you currently require a GEM80 programmer this is the option to choose. The latest version is currently version 5.4, use the hyperlink to access their web site and gain full details of this programmer.
  • Advanced Technical Software Ltd’s latest Windows based GEM80 programming package. Is produced in conjunction with Alstom. It can read all formats of previous programmers including GPP, emulator and Gemesys. The AGP is now sold by both Alstom and ATS and is sold as the current programming package for GEM80 systems.

P80 Pilot


  • The GE popular programmer used in the steel industry is P80 Pilot and is the program adopted by GE. The advantage of P80 Pilot over other programmers is the fact that a project can contain many GEM80 Controllers as well as drives and RXi controllers.

RXI the replacement for GEM80

Finally GEM80’s have not been manufactured for a number of years, so spares are quite scarce. As a result GE have created a replaced for the GEM80 named the RXi. This modern RXi controller can be used with many different types of I/O. These I/O types include GEM80 I/O, Ethercat and WAGO I/O. 

Also legacy GEM80 program can be take from an old GEM80 PLC and converted with some engineering into a GEM program that can be placed into a RXi. This ladder program in the RXi is edited and viewed with P80 Pilot GE program package.


Related Articles