Pseudo-operand initialization
Pseudo-operands are placeholders for instruction backing tag structure members.
IMPORTANT:
When you use an instruction backing tag for a safety-critical operation, you must initialize the pseudo-operands during first scan. Refer to the GuardLogix 5580 and Compact GuardLogix 5380 Controller Systems Safety Reference, publication 1756-RM012, for methods to initialize pseudo-operands during first scan.
Pseudo-operands are initialized when the application is downloaded and never again, unless modified by the application.
- When you specify a pseudo-operand value it is written directly to the member. If you use the same backing tag in another instruction and specify a different value, the previous value is overwritten by the new value.
- Position (POS) and Accumulator (ACC) are initialized as described but they are also overwritten by the instruction when it executes.For example:
- A false Timer on delay (TON) instruction sets the ACC to 0.
- A true TON calculates the elapsed time and adds this to the ACC.
- Preset (PRE) is used by the TON to determine when the DN bit should be set. The instruction does not change the PRE member.
- When a LIFO Load (LFL) instruction executes (false-to-true transition), the source value is written to the LIFO and the POS is incremented.
- When a LIFO Unload (LFU) instruction executes, the value at array[POS] is read and the POS is decremented.
This table lists the pseudo-operands.
TIP:
ASCII Serial Port instructions (AWT, AWA, ARD, ARL, ABL, ACB, AHL, and ACL) are available only on controllers that have serial ports. Logix Designer versions 37 and later do not support ASCII Serial Port instructions.
Instructions | Pseudo-operands | Allowed in safety routines |
ASCII Test for Buffer Line (ABL) | POS | No |
ASCII Chars in Buffer (ACB) | POS | No |
ASCII Handshake Lines (AHL) | POS | No |
ASCII Read (ARD) | LEN, POS | No |
ASCII Read Line (ARL) | LEN, POS | No |
File Average (AVE) | LEN, POS | No |
ASCII Write Append (AWA) | LEN, POS | No |
ASCII Write (AWT) | LEN, POS | No |
Bit Shift Left (BSL) | LEN | No |
Bit Shift Right (BSR) | LEN | No |
Count Up (CTU) | PRE, ACC | Yes |
Count Down (CTD) | PRE, ACC | Yes |
Diagnostic Detect (DDT) | LEN, POS | No |
File Arithmetic and Logic (FAL) | LEN, POS | Yes |
File Bit Comparison (FBC) | LEN, POS | No |
FIFO Load (FFL) | LEN, POS | No |
FIFO Unload (FFU) | LEN, POS | No |
File Search and Compare (FSC) | LEN, POS | Yes |
LIFO Load (LFL) | LEN, POS | No |
LIFO Unload (LFU) | LEN, POS | No |
Retentive Timer On (RTO) | PRE, ACC | Yes |
Sequencer Input (SQI) | LEN, POS | No |
Sequencer Load (SQL) | LEN, POS | No |
Sequencer Output (SQO) | LEN, POS | No |
File Sort (SRT) | LEN, POS | No |
File Standard Deviation (STD) | LEN, POS | No |
Timer Off Delay (TOF) | PRE, ACC | Yes |
Timer On Delay (TON) | PRE, ACC | Yes |
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