Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

24.1.1 Data Files

 

· In brief PLC memory works like the memories in a pocket calculator. The values below are for a PLC-5, although most Allen-Bradley PLCs have a similar structure.

 

 

· These memory locations are typically word oriented (16 bits, or 2 bytes). This includes the bit memory. But the T4, C5, R6 data files are all three words long.

 

· All values are stored and used as integers (except when specified, eg. floating point). When integers are stored in binary format 2's complements are used to allow negative numbers. BCD values are also used.

 

· There are a number of ways the PLC memory can be addressed,

  1. bit - individual bits in memory can be accessed - this is like addressing a single output as a data bit
  2.  
  1. word/integer - 16 bits can be manipulated as a group
  2.  
  1. data value - an actual data value can be provided
  2.  
  1. file level - an array of data values can be manipulated and operated on as a group
  2.  
  1. indirect - another memory location can be used in the description of a location.
  2.  
  1. expression - a text string that describes a complex operation
  2.  

 

· For the user assigned data files from 9 to 999 different data types can be assigned. These can be one of the data types already discussed, or another data type.

  1. A - ASCII
  2. B - bit
  3. BT - block transfer
  4. C - counter
  5. D - BCD
  6. F - floating point
  7. MG - message
  8. N - integer (signed, unsigned, 2s compliment, BCD)
  9. PD - PID controller
  10. R - control
  11. SC - SFC status
  12. ST - ASCII string
  13. T - timer

 

 

24.1.1.1 - Inputs and Outputs

24.1.1.2 - User Bit Memory

24.1.1.3 - Timer Counter Memory

24.1.1.4 - PLC Status Bits (for PLC-5s and Micrologix)

24.1.1.5 - User Function Control Memory

24.1.1.6 - Integer Memory

24.1.1.7 - Floating Point Memory