Programming Cyclic Pump Rotation on a Siemens S7-200 CPU 224 XP

This reference designs and programs a four-pump pressure-boosting station using a SIMATIC S7-200 CPU 224 XP (order number 6ES7 214-1HG40-0XB0) and an EM231 analog input module (6ES7 231-4HD32-0XB0). Pump 1 acts as the primary (jockey) pump between 2.75 bar and 4.0 bar. Pumps 2, 3, and 4 are auxiliary pumps that run two at a time between 2.0 bar and 4.6 bar, rotating selection to balance running hours, and automatically falling back when any pump is in trip state.

All ladder logic shown is targeted at STEP 7 Micro/WIN V4.0 SP9 and follows the conventions in the SIMATIC S7-200 Programmable Controller System Manual (Siemens Support entry 1109582) and the STEP 7 Micro/WIN V4.0 Programming Reference (Siemens Support entry 18849126).

System Overview and Control Objectives

The pumping station must satisfy four independent but interlocking control rules:

1. Maintain system pressure using Pump 1 alone when demand is light, switching ON at 2.75 bar and OFF at 4.0 bar.
2. When pressure falls to 2.0 bar with Pump 1 already running at full output, start two of the three auxiliary pumps (Pumps 2, 3, 4).
3. Stop the running pair when system pressure reaches 4.6 bar. The next low-pressure event must select a different pair of auxiliary pumps to equalize wear.
4. If any auxiliary pump is in trip (motor overload, breaker open, drive fault), exclude it from the selection and force the remaining two healthy pumps to run until the tripped unit is restored.

The pairing of three pumps taken two at a time gives three possible combinations: (2,3), (3,4), and (2,4). Cyclic rotation through these three pairs with a cooldown interval between cycles is the simplest field-proven equalization strategy for small pump stations. A larger plant typically uses a least-hours-start-first strategy; the same ladder scaffold applies, with the comparator block replaced by a sort of running-hour registers.

Hardware Selection and DIP Switch Settings

The CPU 224 XP (6ES7 214-1HG40-0XB0) provides 14 digital inputs, 10 digital outputs, two onboard 12-bit analog inputs (0-10 V only) and one onboard analog output. Because the pressure transmitter is a 4-20 mA device, an EM231 module is required.

Module	Order Number	Function
S7-200 CPU 224 XP DC/DC/DC	6ES7 214-1HG40-0XB0	CPU, 14 DI / 10 DO, 20 KB program memory, 24 V DC supply
EM231 Analog Input	6ES7 231-4HD32-0XB0	4 AI, 12-bit resolution, configurable voltage or current ranges via DIP switches

EM231 4HD32 DIP Switch Configuration

Each EM231 4HD32 has two DIP switch banks of three switches each. Bank A (SW1-SW3) sets the range for channels 0 and 1; Bank B (SW4-SW6) sets the range for channels 2 and 3. Always power-cycle the module after changing DIP switches.

Range	SW1/SW4	SW2/SW5	SW3/SW6	Resolution
0 to 10 V	OFF	OFF	OFF	2.5 mV
0 to 5 V	ON	OFF	OFF	1.25 mV
0 to 1 V	OFF	ON	OFF	250 µV
±10 V	OFF	OFF	ON	2.5 mV
±5 V	ON	OFF	ON	1.25 mV
±2.5 V	OFF	ON	ON	1.25 mV
0 to 20 mA	ON	ON	OFF	5 µA
±20 mA	ON	ON	ON	5 µA

For a 4-20 mA pressure transmitter, set Bank A to ON, ON, OFF (0-20 mA hardware range). The 4 mA zero offset is subtracted in software. Apply this to channels 0 and 1 if redundant transmitters are wired; otherwise only SW1-SW3 are set.

Sensor Wiring and Scaling

Wire the two-wire pressure transmitter to EM231 channel 0 as follows: transmitter positive terminal to the CPU 24 V DC sensor supply (L+ terminal on the EM231 or the CPU sensor supply), transmitter negative terminal to EM231 terminal I+ (channel 0), and short I- to M on the EM231. The CPU 224 XP sensor supply can deliver up to 280 mA at 24 V DC for the entire station, per the S7-200 system manual chapter on sensor supply.

Raw Count to Bar Conversion

The EM231 returns integer counts in the range 0 to 32000 for a 0-20 mA input:

• 4 mA = 6400 counts = 0 bar
• 20 mA = 32000 counts = 10 bar (transmitter full scale)
• Resolution: 1 count = (10 - 0) bar / (32000 - 6400) counts = 0.00390625 bar/count

Inline scaling formula: P_bar = (AIW0 - 6400) / 2560

Equivalent in real-number form for ladder use: P_bar = (AIW0 - 6400) × 0.000390625

I/O Allocation Table

Address	Type	Function	Notes
AIW0	Input (Word)	Pressure transmitter, EM231 channel 0	4-20 mA mapped to 0-10 bar
I0.0	Input	Pump 1 running feedback	Auxiliary contact of contactor
I0.1	Input	Pump 2 running feedback	—
I0.2	Input	Pump 3 running feedback	—
I0.3	Input	Pump 4 running feedback	—
I0.4	Input	Pump 1 trip (overload / breaker)	NC contact, inverted in PLC
I0.5	Input	Pump 2 trip	NC contact, inverted in PLC
I0.6	Input	Pump 3 trip	NC contact, inverted in PLC
I0.7	Input	Pump 4 trip	NC contact, inverted in PLC
I1.0	Input	Auto / Manual selector	0 = Auto, 1 = Manual
I1.1	Input	System enable (E-Stop OK)	Closed = enabled
Q0.0	Output	Pump 1 starter coil	Through contactor
Q0.1	Output	Pump 2 starter coil	—
Q0.2	Output	Pump 3 starter coil	—
Q0.3	Output	Pump 4 starter coil	—
Q0.4	Output	Common fault lamp	—

Variable Memory Map

Address	Type	Function
VD100	REAL	Scaled pressure in bar (0.0 - 10.0)
VB200	BYTE	Auxiliary state machine state (0-4)
VB201	BYTE	Pair rotation counter (0, 1, 2 cycles 3 pairs)
VB202	BYTE	Cooldown timer preset (seconds)
M0.0	BOOL	Pump 1 run latch
M0.1	BOOL	Auxiliary pair demand (pressure < 2.0 bar)
M0.2	BOOL	Auxiliary pair stop (pressure > 4.6 bar)
M0.3	BOOL	Trip detected on Pump 2
M0.4	BOOL	Trip detected on Pump 3
M0.5	BOOL	Trip detected on Pump 4
VD300	DINT	Pump 2 running time accumulator (seconds)
VD304	DINT	Pump 3 running time accumulator (seconds)
VD308	DINT	Pump 4 running time accumulator (seconds)

State Machine for Auxiliary Pumps

The auxiliary pump control is a finite state machine with five states. The state variable is VB200 and the transition rules are evaluated every scan.

Ladder Logic Implementation

Network 1 — Read Raw AIW and Convert to Real

|       SM0.0          MOVW   AIW0,    VW100
|       SM0.0          ITD    VW100,   VD102
|       SM0.0          DTR    VD102,   VD106   // VD106 = AIW0 as REAL

Network 2 — Apply 4 mA Offset and Scale to Bar

|       SM0.0          MOVR   6400.0,  VD110
|       SM0.0          -R     VD106,   VD110   // VD110 = AIW0 - 6400
|       SM0.0          MOVR   0.000390625, VD114
|       SM0.0          *R     VD114,   VD110   // VD110 = pressure in bar

Network 3 — Threshold Comparators with Hysteresis

|       VD110 >= 2.75                  M0.0     // Pump 1 demand (set)
|       M0.0   AND  VD110 < 4.0        (continue latch)
|       M0.0                              Q0.0   // Pump 1 starter

|       VD110 < 2.0                    M0.1     // Aux pair demand
|       VD110 >= 4.6                   M0.2     // Aux pair stop

Network 4 — Trip Status with NC Inversion

|       AN    I0.5         =     M0.3   // Pump 2 trip (NC input)
|       AN    I0.6         =     M0.4   // Pump 3 trip (NC input)
|       AN    I0.7         =     M0.5   // Pump 4 trip (NC input)
|       O     M0.3
|       O     M0.4
|       O     M0.5                =     Q0.4   // Common fault lamp

Network 5 — Pair Selection by Rotation Counter

The pair-rotation counter VB201 cycles through 0, 1, 2 on each high-pressure stop. Use a comparator block to decode the counter to discrete pair select bits.

|       VB201 = 0          =     M10.0   // Select pair A: Pumps 2 and 3
|       VB201 = 1          =     M10.1   // Select pair B: Pumps 3 and 4
|       VB201 = 2          =     M10.2   // Select pair C: Pumps 2 and 4

Network 6 — Trip Override Resolves Pump Selection

|       M10.0 AND NOT M0.3 AND NOT M0.4   =  M11.0   // Pair A healthy
|       M10.1 AND NOT M0.4 AND NOT M0.5   =  M11.1   // Pair B healthy
|       M10.2 AND NOT M0.3 AND NOT M0.5   =  M11.2   // Pair C healthy

// If the selected pair contains a tripped pump, fall back to the next healthy pair
|       M11.0                              =  M12.0
|       M11.1                              =  M12.1
|       M11.2                              =  M12.2

// Trip-fallback: if Pump 2 is tripped, force the system to run Pumps 3 and 4 (the only healthy pair)
|       M0.3   AND  NOT M0.4  AND  NOT M0.5   =  M12.1   // Force pair B
|       M0.4   AND  NOT M0.3  AND  NOT M0.5   =  M12.0   // Force pair A (Pumps 2 and 4)
|       M0.5   AND  NOT M0.3  AND  NOT M0.4   =  M12.0   // Force pair A (Pumps 2 and 3)

Network 7 — State Machine Logic

// Transition S0 -> S1 (start pair A on low pressure)
|       VB200 = 0  AND  M0.1  AND  M12.0  MOVB  1,  VB200

// Transition S1 -> S2 (high pressure reached)
|       VB200 = 1  AND  M0.2                MOVB  2,  VB200

// Increment rotation counter on entering S2
|       VB200 = 1  AND  M0.2                INCB  VB201
|                                  WRB    VB201, VB201  // keep in 0..2
|       VB201 > 2                          MOVB  0,  VB201

// S2 -> S3 after 5 s
|       VB200 = 2  AND  T37                MOVB  3,  VB200

// S3 -> S4 on high pressure
|       VB200 = 3  AND  M0.2                MOVB  4,  VB200

// S4 -> S0 after 5 s (or -> S1 on demand with low pressure)
|       VB200 = 4  AND  T38                MOVB  0,  VB200
|       VB200 = 4  AND  M0.1                MOVB  1,  VB200

Network 8 — Cooldown Timer

|       VB200 = 2  TON  T37, +50         // 5.0 s at 100 ms time base
|       VB200 = 4  TON  T38, +50

Network 9 — Pump Output Drive

|       (VB200 = 1  AND  M12.0)
|       (VB200 = 1  AND  M12.1)
|       (VB200 = 3  AND  M12.1)
|       (VB200 = 3  AND  M12.2)
|       (VB200 = 0  AND  M0.1  AND  (M12.0 OR M12.1 OR M12.2))
|  =    Q0.1    // Pump 2

|       (VB200 = 1  AND  M12.0)
|       (VB200 = 1  AND  M12.1)
|       (VB200 = 3  AND  M12.1)
|       (VB200 = 3  AND  M12.2)
|       (VB200 = 0  AND  M0.1  AND  (M12.0 OR M12.1 OR M12.2))
|  =    Q0.2    // Pump 3

|       (VB200 = 3  AND  M12.1)
|       (VB200 = 3  AND  M12.2)
|       (VB200 = 4  AND  M0.1  AND  (M12.1 OR M12.2))
|  =    Q0.3    // Pump 4

Network 10 — Minimum Run and Minimum Off Time (Anti-Cycle)

To prevent rapid cycling and contactor wear, latch each auxiliary output for a minimum of 30 seconds on time and 30 seconds off time.

|       Q0.1  OR  (M0.1  AND  VB200 = 1  AND  M12.0)  TON  T101, +300
|       T101  S    M30.1                                 // Pump 2 minimum run
|       M30.1 = Q0.1

|       NOT Q0.1  TON  T102, +300
|       T102     R    M30.1

Repeat the same ladder block for Pumps 3 and 4 with timers T103, T104 and T105, T106, and latches M30.2, M30.3, M30.4.

Running Hours Tracking

Use the 1-minute clock bit SM0.4 to accumulate running time. The S7-200 system manual documents SM0.4 as a 1-minute clock with 50% duty cycle. Each rising edge of SM0.4 therefore represents 30 seconds of operation; doubling the count or storing directly in minutes is acceptable.

|       SM0.4
|       EU                  // Detect rising edge once per minute
|       A     Q0.1          INCD  VD300    // Pump 2 minutes
|       A     Q0.2          INCD  VD304    // Pump 3 minutes
|       A     Q0.3          INCD  VD308    // Pump 4 minutes

To display running hours, divide each accumulator by 60. In the HMI or SCADA tag, expose VD300 / 60, VD304 / 60, and VD308 / 60 as integer hours. Retain values in the permanent memory area (VB14 to VB31, controlled by SMB31 and SMB32) so the counters survive power loss.

Trip and Fault Handling

The trip-fallback logic in Network 6 guarantees that a single tripped auxiliary pump does not stall the system. If two auxiliary pumps are tripped simultaneously, the system cannot meet the design pressure band. The PLC must raise the common fault lamp and write a distinct fault code to memory for the HMI:

Fault Code	Condition	Action
0	No fault	—
1	One auxiliary pump tripped	Fallback to remaining two healthy pumps
2	Two auxiliary pumps tripped	Latch fault, run Pump 1 only, raise alarm, force common fault lamp
3	All three auxiliary pumps tripped	Stop all pumps except Pump 1, raise critical alarm, disable auto-restart
10	Pressure transmitter open loop (AIW0 < 6000)	Hold last valid pressure, raise sensor fault
11	Pressure transmitter over-range (AIW0 > 32500)	Clamp to 10 bar, raise sensor fault

Trip Auto-Reset Logic

Once the operator clears the trip condition at the motor protection breaker, the PLC should immediately accept the restored pump. Implement this with a positive edge on the trip input:

|       AN    I0.5  EU    M40.1    // Pump 2 trip clears
|       M40.1                R     M0.3

|       AN    I0.6  EU    M40.2    // Pump 3 trip clears
|       M40.2                R     M0.4

|       AN    I0.7  EU    M40.3    // Pump 4 trip clears
|       M40.3                R     M0.5

Commissioning Procedure

1. Power the S7-200 station with all motor starters disconnected. Verify the CPU enters RUN mode and the SF/DIAG LED is OFF.
2. Open STEP 7 Micro/WIN V4.0 SP9, go online, and confirm the EM231 is recognized in the system block.
3. Force AIW0 to 6400 with the transmitter disconnected. Verify VD110 reads 0.0 bar (within ±0.01 bar).
4. Apply a 4 mA calibration signal. Verify VD110 reads 0.0 bar.
5. Apply a 12 mA signal. Verify VD110 reads 5.0 bar ±0.02 bar.
6. Apply a 20 mA signal. Verify VD110 reads 10.0 bar ±0.02 bar.
7. Force Q0.0 in the status chart. Verify Pump 1 contactor pulls in. Reset.
8. Force Q0.1, Q0.2, Q0.3 in turn. Verify each pump contactor pulls in. Reset.
9. Force M0.1 (aux pair demand). Verify the state machine transitions S0 → S1 and the correct pair drives the outputs.
10. Force M0.2 (aux pair stop). Verify the state machine transitions to the cooldown state, then the next pair.
11. Simulate a trip on Pump 2 by opening I0.5. Verify that Pumps 3 and 4 run continuously and the fault lamp Q0.4 turns ON.
12. Restore I0.5. Verify the state machine returns to normal rotation on the next demand cycle.
13. Disconnect the pressure transmitter. Verify the PLC holds the last valid pressure and raises the sensor fault code.

Verification Checks

Test	Acceptance Criterion
Pump 1 turn-on pressure	2.70 - 2.80 bar
Pump 1 turn-off pressure	3.95 - 4.05 bar
Auxiliary turn-on pressure	1.95 - 2.05 bar
Auxiliary turn-off pressure	4.55 - 4.65 bar
Pair rotation	Sequential: (2,3) → (3,4) → (2,4) → repeat
Cooldown duration	5.0 s ±0.5 s
Minimum run time	30 s ±1 s
Single-trip response	Within 1 s of trip input, healthy pair runs
Running-hour resolution	1 minute increments

Troubleshooting Matrix

Symptom	Likely Cause	Diagnostic Step	Corrective Action
All pumps run continuously	Pressure transmitter open, AIW0 = 0	Monitor AIW0 in status chart	Check loop wiring, replace transmitter
Pump 1 chatters	Pressure band too narrow or transmitter noise	Read VD110 with trend; check electrical noise	Increase hysteresis to 2.65-4.10, add 0-5 s input filter
No pumps start	System enable input open or all aux tripped	Check I1.1 and M0.3-M0.5	Restore enable, clear trips
Aux pair never rotates	VB201 not incrementing	Watch VB201 in status chart during high pressure	Verify M0.2 is asserted when VD110 ≥ 4.6
Same two pumps always run	Trip input stuck on third pump	Force I0.5-I0.7 and watch M0.3-M0.5	Reset overload device; check wiring polarity of NC contact
VD110 reads 0.0 bar at known pressure	EM231 DIP switches in voltage mode	Power down, inspect SW1-SW3	Set SW1=ON, SW2=ON, SW3=OFF
VD110 reads -0.5 bar at 0 bar	Wrong 4 mA zero offset	Check raw AIW0 at 4 mA	Re-trim transmitter or adjust offset constant
SF LED ON after power-up	EM231 not detected	Check expansion bus connection	Reseat module; verify unique I/O addressing

Edge Cases and Field-Proven Caveats

• Water hammer: never stop two pumps simultaneously on the same high-pressure event. The 5-second cooldown staggers the stop and prevents surge. For long pipe runs, increase the cooldown to 10-15 s.
• Sensor drift: the 4 mA zero of industrial pressure transmitters drifts with temperature. Schedule a six-monthly two-point calibration against a dead-weight tester, and adjust the 6400 offset constant in VD110 if the zero error exceeds ±0.05 bar.
• Power-on reset: the S7-200 retains the V memory only if the retentive range is configured in the system block. The default VB0-VB1023 is volatile. Mark VD300, VD304, and VD308 as retentive in the System Block > Retentive Ranges dialog.
• Scan time impact: the floating-point math in Network 2 takes roughly 12 µs per scan on a CPU 224 XP. With a typical scan of 5 ms, this is negligible. Do not place the scaling math inside a high-speed counter or interrupt routine.
• EMI on the 4-20 mA loop: route the analog cable in a separate conduit at least 200 mm from VFD output cables. If the transmitter is more than 50 m from the PLC, install a 250 Ω resistor at the PLC terminal and convert to 1-5 V (DIP set to 0-5 V, scale formula becomes P = (AIW0 - 640) / 2560).
• Multiple transmitters: the EM231 4HD32 has only 4 channels. If redundancy is required, install a second EM231 and use the higher of the two pressures in the control logic.

Frequently Asked Questions