Programming languages Statement list Function block Ladder diagram The instructions are represented by graphic symbols: Contacts, Coils & Boxes The ladder diagram is the most popular programming language
Instructions Standard instructions: They are used in most programs. Examples: timer, counter, math, logical, incr., decr. and move High speed instructions: They allow for events and interrupts to occur independently of the PLC scan time. Examples: High speed counters and interrupts Special instructions: They are used to manipulate data Shift, table, conversion, real time instruction .
Bit Logic instruction Normally Open contact Normally Open Immediate contact Normally Closed contact Not contact Normally Closed Immediate contact Positive Transition contact Negative Transition contact Input Instructions
On-Delay & Retentive On-Delay timers They count time when the enabling input (IN) is ON. When the current value (Txxx) is > the preset time (PT), the timer bit is ON. The On-Delay timer current value is cleared when (IN) is OFF, while the current value of the Retentive On-Delay Timer is maintained. You can use the Retentive On-Delay Timer to accumulate time for multiple periods of the input ON.
Off-Delay timer The Off-Delay Timer is used to delay turning an output OFF for a fixed period of time after the input turns OFF. When (IN) turns ON, the timer bit turns ON immediately, and the current value is set to 0. When (IN) turns OFF, the timer counts till PT and the timer bit turns OFF and the current value stops counting. If the input is OFF for a time shorter than PT, the timer bit remains ON.
Timers numbers & resolutions Note You cannot share the same timer numbers for TOF and TON. For example, you cannot have both a TON T32 and a TOF T32.
It counts up on the rising edges of the Count Up (CU) input.
When the current value (Cxxx) > (PV), the counter bit (Cxxx) turns on.
The counter is reset when the Reset (R) input turns on.
Up/Down counter It counts up on rising edges of the Count Up (CU) input. It counts down on the rising edges of the Count Down (CD) input. When the current value (Cxxx) > (PV), the counter bit (Cxxx) turns on. The counter is reset when the Reset (R) input turns on.
Down counter It counts down from the PV on the rising edges of the (CD) input . When the current value is equal to zero, the counter bit (Cxxx) turns on. The counter resets the counter bit (Cxxx) and loads the current value with the (PV) when the load input (LD) turns on.
Counter example A counter might be used to keep track of the number of vehicles in a parking lot. As vehicles enter the lot through an entrance gate, the counter counts up. As vehicles exit the lot through an exit gate, the counter counts down. When the lot is full a sign at the entrance gate turns on indicating the lot is full.
They are used to organize machine operations or steps into equivalent program segments. SCRs allow logical segmentation of the control Format: Bit S[byte address].[bit address] S3.1 Byte, Word, Double Word S[size][starting byte address] SB4
Special memory bits (SM)
The SM bits provide a means for communicating information between the CPU and your program. You can use these bits to select and control some of the special functions of the S7-200 CPU, such as: • A bit that turns on for the first scan cycle • Bits that toggle at fixed rates • Bits that show the status of math or operational instructions Format: Bit SM[byte address].[bit address] SM0.1 Byte, Word, Double Word SM[size][starting byte address] SMB86
The S7-200 PLCs provide 64 bytes of local (L) memory of which 60 can be used as scratchpad memory or for passing formal parameters to subroutines. Format: Bit L [byte address].[bit address] L0.0 Byte, Word, Double Word L [size] [starting byte address] LB33
The S7-200 converts a real-world, analog value (such as temperature or voltage) into a word-length (16-bit) digital value. You access these values by the area identifier (AI), size of the data (W), and the starting byte address. Since analog inputs are words and always start on even-number bytes (such as 0, 2, or 4), you access them with even-number byte addresses (such as AIW0, AIW2, or AIW4),as shown in Figure Analog input values are read-only values. Format: AIW [starting byte address] AIW4
Memory types The S7-200 converts a word-length (16-bit) digital value into a current or voltage, proportional to the digital value (such as for a current or voltage). You write these values by the area identifier (AQ), size of the data (W), and the starting by address. Since analog outputs are words and always start on even-number bytes (such as 0, 2, or 4), you write them with even-number byte addresses (AQW0, AQW2, AQW4), Format: AQW [starting byte address] AQW4
Analog outputs (AQ)
Move instructions The Move Byte instruction moves the input byte (IN) to the output byte (OUT). The input byte is not altered by the move. The Move Word instruction moves the input word (IN) to the output word (OUT). The input word is not altered by the move. The Move Double Word instruction moves the input double word (IN) to the output double word (OUT). The input double word is not altered by the move. The Move Real instruction moves a 32-bit, real input double word (IN) to the output double word (OUT). The input double word is not altered by the move.
The Block Move Byte instruction moves the number of
bytes (N) from the input address IN to the output address
OUT. N has a range of 1 to 255.
Move byte immediate instructions The Move Byte Immediate Write instruction reads from location IN and writes to physical output OUT. The Move Byte Immediate Read instruction reads physical input IN and writes the result in OUT.
Analogue I/O = Typical analogue signals from 0-10 VDC or 4-20 mA = They are used to represent changing values such as speed, temperature, weight and level =The expansion module converts the standard voltage and current values to 12-bit digital representation. These digital values are transferred to the PLC for use in its program