PLC adressing
- Thread starter bpenbert
- Start date
PhilipW
Member
It all SEEMS rather hard until you understand the reason WHY.
Modern PLC's like the SLC500 and ControlLogix simply create I/O image datatable to match the physical I/O attached to the processor. But in the days of the early PLC's, PLC, PLC2, PLC3 and PLC5 the I/O image in the processor was FIXED in size to a certain number of 16 bit words.
Typically a processor was specified with 8, 16 , 32 , 64 racks of I/O image. A rack is 8 words of image, each word being 16 bits long.
The 1771 I/O system was originally designed for 8 point modules. This meant that in order to make efficient use of the processor I/O image, that TWO 8 point modules (adjacent in the 1771 chassis) would be mapped onto ONE 16 bit I/O word in the image.
This was just the normal way of doing things for a few years until along came 16 point modules. In order to allow the 1771 I/O system to stay in business, AB then introduced the idea of 2-slot and 1-slot addressing.
This meant that a single 16 point module would be mapped onto ONE 16 bit I/O word in the processor. The selection was made by a DIP switch on the chassis; in other words the WHOLE chassis was either in 2-slot or 1-slot mode.
If you had a mix of 16 point and 8 point modules you had to select 1 slot addressing to accomodate the 16 point modules, and just accept that the 8 point modules left unsused space in the I/O image.
When 32 point modules finally became available the concept was just extended to "1/2-slot addressing" wherein ONE 32 bit module would be mapped onto TWO adjacent 16 bit words in the processor I/O image; or to put it the other way around, each ONE word in the I/O image addressed only 1/2 of a 32 point I/O module.
An entirely separate issue called "complementary addressing" was another clever ruse to make even more efficient use of the processor I/O image. All up ...configuring 1771 I/O chassis and racks required specific knowledge of what you were doing.
The upside of this system is that AB were able to maintain the life and protect their customers investment in a huge installed base of 1771 I/O systems.... long after their competitors had to migrate to completely new I/O systems.
Although after almost 25 years of commercial life, no-one would describe 1771 I/O as leading edge system, plenty of it is still in use and still sold, largely because of the flexibility that the 2-slot, 1-slot and 1/2-slot addressing scheme introduced into an otherwise inflexible system.
Modern PLC's like the SLC500 and ControlLogix simply create I/O image datatable to match the physical I/O attached to the processor. But in the days of the early PLC's, PLC, PLC2, PLC3 and PLC5 the I/O image in the processor was FIXED in size to a certain number of 16 bit words.
Typically a processor was specified with 8, 16 , 32 , 64 racks of I/O image. A rack is 8 words of image, each word being 16 bits long.
The 1771 I/O system was originally designed for 8 point modules. This meant that in order to make efficient use of the processor I/O image, that TWO 8 point modules (adjacent in the 1771 chassis) would be mapped onto ONE 16 bit I/O word in the image.
This was just the normal way of doing things for a few years until along came 16 point modules. In order to allow the 1771 I/O system to stay in business, AB then introduced the idea of 2-slot and 1-slot addressing.
This meant that a single 16 point module would be mapped onto ONE 16 bit I/O word in the processor. The selection was made by a DIP switch on the chassis; in other words the WHOLE chassis was either in 2-slot or 1-slot mode.
If you had a mix of 16 point and 8 point modules you had to select 1 slot addressing to accomodate the 16 point modules, and just accept that the 8 point modules left unsused space in the I/O image.
When 32 point modules finally became available the concept was just extended to "1/2-slot addressing" wherein ONE 32 bit module would be mapped onto TWO adjacent 16 bit words in the processor I/O image; or to put it the other way around, each ONE word in the I/O image addressed only 1/2 of a 32 point I/O module.
An entirely separate issue called "complementary addressing" was another clever ruse to make even more efficient use of the processor I/O image. All up ...configuring 1771 I/O chassis and racks required specific knowledge of what you were doing.
The upside of this system is that AB were able to maintain the life and protect their customers investment in a huge installed base of 1771 I/O systems.... long after their competitors had to migrate to completely new I/O systems.
Although after almost 25 years of commercial life, no-one would describe 1771 I/O as leading edge system, plenty of it is still in use and still sold, largely because of the flexibility that the 2-slot, 1-slot and 1/2-slot addressing scheme introduced into an otherwise inflexible system.
randylud
Member
Great Job!
PhilipW, that has to be one of the best explanations of AB 1771 I/O addressing I have ever encountered.
It is clear, concise, and easy to understand. As a matter of fact, I have downloaded the thread and plan on using it in my PLC class this term. Good show, mate!
PhilipW, that has to be one of the best explanations of AB 1771 I/O addressing I have ever encountered.
It is clear, concise, and easy to understand. As a matter of fact, I have downloaded the thread and plan on using it in my PLC class this term. Good show, mate!Similar Topics
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