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.
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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