Purpose
Function block for managing 4-20mA analog inputs with error detection and automatic recovery. Designed for SIMATIC S7-315 systems.
Hardware Requirements
- Module: SIMATIC ET 200S 2AI I 4WIRE ST (6ES7 134-4GB11-0AB0)
- Range: 4-20mA
- Connection: 4-wire configuration
Interface
Input
| Parameter |
Type |
Description |
i_IW_Analog |
WORD |
Raw analog input value (PEW) |
Outputs
| Parameter |
Type |
Description |
o_CurrentMa |
REAL |
Current value in mA |
o_ErrorCode |
INT |
Error code (0=OK, higher values = higher severity) |
Error Codes (Priority Order)
| Code |
Severity |
Description |
Current Output |
| 0 |
OK |
Normal range (4-20mA) |
Calculated value |
| 1 |
WARNING |
Overload range (20-22.8mA) |
Calculated value |
| 2 |
WARNING |
Underload range (3.9995-4mA) |
Calculated value |
| 10 |
ERROR |
Overflow (>22.8mA) |
22.8142mA |
| 11 |
ERROR |
Underflow (<3.9995mA) |
3.9995mA |
| 20 |
CRITICAL |
Wire Break (32767 value) |
22.8142mA |
| 99 |
CRITICAL |
Undefined range |
3.9995mA |
Error Recovery Behavior
Cooldown Mechanism
- When a critical error (code ≥10) is resolved, a 500ms cooldown period begins
- During cooldown: previous error code and current value are maintained
- After cooldown: automatic reset to normal operation
- No manual reset required
Current Value Logic
- Normal operation: Calculated using linear conversion:
mA = 4 + (AnalogValue / 27648) * 16
- Error conditions: Fixed logical values based on error type
- Value limits: Always constrained between 3.9995mA and 22.8142mA
Usage Example
// Instance declaration
VAR
AnalogInput_Measurement : FB_AnalogMeasurement;
InputValue : WORD;
CurrentMa : REAL;
ErrorStatus : INT;
END_VAR
// Function block call
AnalogInput_Measurement(
i_IW_Analog := InputValue, // Connect to PEW address
o_CurrentMa => CurrentMa, // Current in mA
o_ErrorCode => ErrorStatus // Error monitoring
);
// Error handling example
CASE ErrorStatus OF
0: // Normal operation
1,2: // Warnings - operation continues
10,11,20,99: // Critical errors - check wiring/module
END_CASE;
Key Features
- Simplified operation: No diagnostic configuration required
- Automatic recovery: 500ms cooldown with self-reset
- Prioritized error handling: ELSIF logic ensures most critical errors take precedence
- Logical fallback values: Sensible current values during error conditions
- No memory: Stateless operation after cooldown period
// FB2121 - Simplified 4-20mA Analog Measurement Management
// Required hardware configuration:
// - SIMATIC ET 200S 2AI I 4WIRE ST (6ES7 134-4GB11-0AB0)
// - Range: 4-20mA
FUNCTION_BLOCK "FB_AnalogMeasurement"
TITLE = 'Simplified 4-20mA analog input management'
AUTHOR : tk
FAMILY : Process
VAR_INPUT
i_IW_Analog : WORD; // Analog input value (PEW)
END_VAR
VAR_OUTPUT
o_CurrentMa : REAL; // Current value in mA
o_ErrorCode : INT; // Error code (0=OK, increasing priority by severity)
END_VAR
VAR
// Internal variables
AnalogValueInt : INT; // Analog value converted to INT
CurrentErrorCode : INT; // Current error code
PreviousErrorCode : INT; // Previous error code for cooldown
PreviousCurrentMa : REAL; // Previous current value for cooldown
// Timer for 500ms cooldown
CooldownTimer : TON;
CooldownTime : TIME := T#500MS; // 500ms cooldown
InCooldown : BOOL; // Flag indicating if in cooldown
END_VAR
VAR_TEMP
TempReal : REAL; // Temporary value for floating point calculations
END_VAR
CONST
// Constants definition according to 4-20mA analog module documentation
OVERFLOW_VALUE := 32767; // 7FFFH - Overflow/wire break value
UNDERFLOW_VALUE := -32768; // 8000H - Underflow value
UNDERFLOW_DIAG := -32767; // 8001H - Underflow with diagnostics enabled
MAX_NORMAL := 27648; // 6C00H - 20mA (maximum value in nominal range)
MIN_NORMAL := 0; // 0000H - 4mA (minimum value in nominal range)
RANGE_HIGH := 32511; // 7EFFH - 22.8142mA (overload range start)
RANGE_LOW := -1; // FFFFH - 3.9995mA (underload range end)
// Logical values for extreme conditions
OVERFLOW_MA_VALUE := 22.8142; // mA value for overflow
UNDERFLOW_MA_VALUE := 3.9995; // mA value for underflow
END_CONST
BEGIN
// Convert WORD value to INT for processing
AnalogValueInt := WORD_TO_INT(i_IW_Analog);
// ========================================================================================
// ANALOG VALUE ANALYSIS AND ERROR CODE DETERMINATION (ELSIF PRIORITY)
// ========================================================================================
// Error codes by priority (0=OK, increasing numbers = higher severity):
// 0 = OK - Nominal range (4-20mA)
// 1 = WARNING - Overload range (20-22.8mA)
// 2 = WARNING - Underload range (3.9995-4mA)
// 10 = ERROR - Overflow (>22.8mA or 32767)
// 11 = ERROR - Underflow (<3.9995mA or negative values)
// 20 = CRITICAL - Wire Break (32767 confirmed)
// 99 = CRITICAL - Undefined range
// ELSIF list with concurrent error priority (from most severe to least severe)
IF AnalogValueInt = OVERFLOW_VALUE THEN // 32767 (7FFFH) - Most critical
CurrentErrorCode := 20; // Wire Break
ELSIF AnalogValueInt = UNDERFLOW_VALUE OR
AnalogValueInt = UNDERFLOW_DIAG THEN // -32768 or -32767
CurrentErrorCode := 11; // Critical underflow
ELSIF AnalogValueInt > RANGE_HIGH THEN // > 32511 (overflow)
CurrentErrorCode := 10; // Overflow
ELSIF AnalogValueInt < RANGE_LOW THEN // < -1 (underflow)
CurrentErrorCode := 11; // Underflow
ELSIF AnalogValueInt > MAX_NORMAL AND
AnalogValueInt <= RANGE_HIGH THEN // Overload range
CurrentErrorCode := 1; // Warning overload
ELSIF AnalogValueInt < MIN_NORMAL AND
AnalogValueInt >= RANGE_LOW THEN // Underload range
CurrentErrorCode := 2; // Warning underload
ELSIF AnalogValueInt >= MIN_NORMAL AND
AnalogValueInt <= MAX_NORMAL THEN // Nominal range
CurrentErrorCode := 0; // OK
ELSE // Any other case
CurrentErrorCode := 99; // Undefined range
END_IF;
// ========================================================================================
// 500MS COOLDOWN MANAGEMENT
// ========================================================================================
// Detect transition from error to no-error to start cooldown
IF PreviousErrorCode >= 10 AND CurrentErrorCode < 10 THEN
InCooldown := TRUE;
CooldownTimer(IN := TRUE, PT := CooldownTime);
END_IF;
// Manage cooldown timer
CooldownTimer(IN := InCooldown, PT := CooldownTime);
// End cooldown when timer expires
IF CooldownTimer.Q THEN
InCooldown := FALSE;
CooldownTimer(IN := FALSE, PT := CooldownTime);
END_IF;
// ========================================================================================
// ERROR CODE AND CURRENT VALUE ASSIGNMENT
// ========================================================================================
// If in cooldown, maintain previous error and current
IF InCooldown THEN
o_ErrorCode := PreviousErrorCode;
o_CurrentMa := PreviousCurrentMa;
ELSE
// Assign current error code
o_ErrorCode := CurrentErrorCode;
// Assign current value based on detected error
IF CurrentErrorCode = 20 OR CurrentErrorCode = 10 THEN // Wire Break or Overflow
o_CurrentMa := OVERFLOW_MA_VALUE; // 22.8142mA
ELSIF CurrentErrorCode = 11 THEN // Underflow
o_CurrentMa := UNDERFLOW_MA_VALUE; // 3.9995mA
ELSIF CurrentErrorCode = 99 THEN // Undefined range
o_CurrentMa := UNDERFLOW_MA_VALUE; // 3.9995mA (safe value)
ELSE
// Normal calculation for OK and WARNING ranges: mA = 4 + (AnalogValueInt / 27648) * 16
TempReal := INT_TO_REAL(AnalogValueInt);
o_CurrentMa := 4.0 + (TempReal / INT_TO_REAL(MAX_NORMAL)) * 16.0;
// Limit to logical maximum and minimum values
IF o_CurrentMa < UNDERFLOW_MA_VALUE THEN
o_CurrentMa := UNDERFLOW_MA_VALUE; // Minimum: 3.9995mA
ELSIF o_CurrentMa > OVERFLOW_MA_VALUE THEN
o_CurrentMa := OVERFLOW_MA_VALUE; // Maximum: 22.8142mA
END_IF;
END_IF;
// Save values for next cycle
PreviousErrorCode := CurrentErrorCode;
PreviousCurrentMa := o_CurrentMa;
END_IF;
END_FUNCTION_BLOCK