Dual-Channel Input Stop with Test (DCST)

This instruction applies to the Compact GuardLogix 5370, GuardLogix 5570, Compact GuardLogix 5380, and GuardLogix 5580 controllers.
The Dual-Channel Input Stop with Test (DCST) instruction monitors dual-input safety devices whose main function is to stop a machine safely, for example, an E-stop, light curtain, or safety gate. This instruction can only energize Output 1 when both safety inputs, Channel A and Channel B, are in the active state as determined by the Input Type operand, and the correct reset actions are carried out.
In addition, this instruction has the ability to force a functional test of the stop device upon request.
The timing diagrams from the Dual-Channel Input Stop (DCS) instruction are applicable to this instruction as well.
DCST operation diagrams in this instruction, highlight the features of the test-related operands such as Test Request and Test Command.
Available Languages
Ladder Diagram
L5K_DCST_LD_avail_v31
Function Block
This instruction is not available in function block.
Structured Text
This instruction is not available in structured text.
Operands
IMPORTANT:
Unexpected operation may occur if:
  • Output tag operands are overwritten.
  • Members of a structure operand are overwritten.
  • Structure operands are shared by multiple instructions.
IMPORTANT:
Make sure safety input points are configured as single, not Equivalent or Complementary. These instructions provide all dual channel functionality necessary for PLd (Cat. 3) or PLe (Cat. 4) safety functions.
WARNING:
ATTENTION:
If changing instruction operands while in Run mode, accept the pending edits and cycle the controller mode from Program to Run for the changes to take effect.
The following table provides the operands that are used to configure the instruction. These operands cannot be changed at runtime.
Operand
Data Type
Format
Description
DCST
DCI_STOP_TEST
tag
DCST structure
Safety Function
DINT
list item
This operand provides a text name for how this instruction is being used. Choices include E-stop, safety gate, light curtain, area scanner, safety mat, cable (rope) pull switch, and user-defined.
This operand does not affect instruction behavior. It is for information/documentation purposes only.
Input Type
DINT
list item
This operand selects input channel behavior.
Equivalent - Active High (0):
 Inputs are in the active state when Channel A and Channel B inputs are 1.
Complementary (2):
 Inputs are in the active state when Channel A is 1 and Channel B is 0.
Discrepancy Time (ms)
DINT
immediate
The amount of time that the inputs can be in an inconsistent state before an instruction fault is generated. The inconsistent state depends on the Input Type.
Equivalent: Inconsistent state is when:
Channel A = 0 and Channel B =1, or
Channel A =1 and Channel B =0
Complementary: Inconsistent state is when:
Channel A = 0 and Channel B =0, or
Channel A =1 and Channel B =1
The range is 5...3000 ms.
Restart Type
BOOL
immediate
This input configures Output 1 for either manual or automatic restart.
Manual (0): -
A transition of the reset input from OFF (0) to ON (1), while all of the Output 1 enabling conditions are met, is required to energize Output 1
Automatic (1): -
Output 1 is energized 50 ms after all of the enabling conditions are met.
Important:
Automatic restart may only be used in application situations where no unsafe conditions can occur as a result of its use, or the reset function is being performed elsewhere in the safety circuit (for example, output function).
Cold Start Type
BOOL
list item
This operand specifies the Output 1 behavior when applying controller power or mode change to Run.
Manual (0): -
Output 1 is not energized when the Input Status becomes valid or when the Input Status fault is cleared. (The device must be tested before Output 1 can be energized.)
Automatic (1): -
Output 1 is energized immediately when the Input Status becomes valid or when the Input Status fault is cleared and both inputs are in their active state.
The following table explains instruction inputs. The inputs may be field device signals from input devices or derived from user logic.
Operand
Data Type
Format
Description
Channel A
1
BOOL
tag
This input is one of the two safety inputs to the instruction.
Channel B
1
BOOL
tag
This input is one of the two safety inputs to the instruction.
Test Request
BOOL
tag
This signal forces a functional test to occur.
ON (1) -> OFF (0): Triggers a functional test. Output 1 is de-energized and the Test Command output is energized, which prompts for a functional test to be performed.
The functional test is complete and the Test Command output is de-energized when Channel A and Channel B go to the safe state.
Input Status
BOOL
immediate
tag
If instruction inputs are from a safety I/O module, this is the status from the I/O module (Connection Status or Combined Status). If instruction inputs are derived from internal logic, it is the application programmer’s responsibility to determine the conditions.
ON (1): The inputs to this instruction are valid.
OFF (0): The inputs to this instruction are invalid.
Reset
2
BOOL
tag
If Restart Type = Manual, this input is used to energize Output 1 once Channel A and Channel B are both in the active state.
If Restart Type = Automatic, this input is used to energize Output 1.
This input clears instruction and circuit faults provided the fault condition is not present.
OFF (0) -> ON (1): The FP (Fault Present) and Fault Code outputs are reset.
1
 If the input is from a
Guard I/O
 input module, make sure that the input is configured as single, not Equivalent or Complementary.
2
 ISO 13849-1 stipulates instruction reset functions must occur on falling edge signals. To comply with ISO 13849-1 requirements, add this logic immediately before this instruction. Rename the Reset_Signal tag in this example to the reset signal tag name. Then use the OSF instruction Output Bit tag as the reset source of the instruction.
Reset_Signal Example_v31
The following table explains the instruction outputs. The outputs can be external tags (safety output modules) or internal tags for use in other logic routines.
Operand
Data Type
Description
Output 1 (01)
BOOL
This output is energized when the input conditions have been satisfied.
The output becomes de-energized when:
  • Either Channel A or Channel B transitions to the safe state.
  • The Input Status is OFF (0).
  • A functional test is requested (Test Request > OFF (0).
Test Command (TC)
BOOL
This output is energized when a functional test must be carried out.
The operand is not safety-related.
Fault Present (FP)
BOOL
ON (1): A fault is present in the instruction.
OFF (0): This instruction is operating normally.
Fault Code
DINT
This output indicates the type of fault that occurred. See the Fault Codes section below for a list of fault codes.
This operand is not safety-related.
Diagnostic Code
DINT
This output indicates the diagnostic status of the instruction. See the Diagnostic Codes section below for a list of diagnostic codes.
This operand is not safety-related.
IMPORTANT:
Do not write to any instruction output tag under any circumstances.
Affects Math Status Flags
No
Major/Minor Faults
None specific to this instruction. See Index Through Arrays for array-indexing faults.
Execution
Condition/State
Action Taken
Prescan
Same as Rung-condition-in is false.
Rung-condition-in is false
The .O1, .TC and .FP are cleared to false.
Rung-condition-in is true
The instruction executes as described in the Operation section.
Postscan
Same as Rung-condition-in is false.
Operation
Functional Test Operation (Manual Restart)
The timing diagram illustrates a manual functional test being performed on a safety device, for example, a safety gate, with the instruction configured for manual restart. At (A), a manual functional test is requested because the Test Request input transitions from ON (1) to OFF (0). This immediately de-energizes Output 1 and energizes the Test Command output, which prompts for a test of the device to be performed. At (B), the functional test is complete, so the Test Command output is de-energized. At (C), Output 1 is energized again when a reset is triggered.
DCST Functional Test Operation_manual restart
Functional Test Operation (Automatic Restart)
The timing diagram illustrates a manual function test being performed with Restart Type equal to Automatic. At (A), Output 1 is de-energized because the Test Request transitions from ON (1) to OFF (0). The Test Command output is also energized at this point. At (B), the Test Command output is de-energized because the functional test is complete. At (C), Output 1 is automatically energized 50 ms after the safety inputs enter the active state because the restart type is automatic.
DCST Functional Test Operation_auto restart
False Rung State Behavior
When the instruction is executed on a false rung, all instruction outputs are de-energized.
Fault Codes and Corrective Actions
Fault Code
Description
Corrective Action
0
No fault.
None.
16#20
32
The Input Status input transitioned from ON (1) to OFF (0) while the instruction was executing.
  • Check the I/O module connection or the internal logic used to source input status.
  • Reset the fault.
16#4000
16384
Channel A and Channel B were in an inconsistent state for longer than the Discrepancy Time. At the time of the fault, Channel A was in the active state. Channel B was in the safe state.
  • Check the wiring.
  • Perform a functional test of the device (put Channel A and Channel B in a safe state).
  • Reset the fault.
16#4001
16385
Channel A and Channel B were in an inconsistent state for longer than the Discrepancy Time. At the time of the fault, Channel A was in the safe state. Channel B was in the active state.
16#4002
16386
Channel A went to the safe state and back to the active state while Channel B remained active.
16#4003
16387
Channel B went to the safe state and back to the active state while Channel A remained active.
Diagnostic Codes and Corrective Actions
Diagnostic Code
Description
Corrective Action
00H
No fault.
None.
16#05
5
The Reset input is held ON (1)
Set the Reset input to OFF (0)
16#20
32
The Input Status was OFF(0) when the instruction started.
Check the I/O module connection or the internal logic used to source input status.
16#4000
16384
The device has not been functionally tested at startup.
Perform a functional test of the inputs (put Channel A and Channel B in a safe state).
16#4001
16385
The device has not been functionally tested after a fault occurs.
  • Check the wiring.
  • Perform a functional test of the device (put Channel A and Channel B in a safe state).
16#4030
16432
Waiting for the manual functional test to occur.
Perform a functional test of the device (put Channel A and Channel B in a safe state).
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