LOGO! 8: Implementing 10-Output Pump Selector with VM Mapping

Overview

Siemens LOGO! 8 logic modules support a compact 1-of-N pump selector in which the operator chooses one of 10 pumps from a LOGO! TDE text display, confirms the selection with F2, and the controller drives a single digital output. The technique described in the field report uses the LOGO!Soft Comfort "Up/Down Counter" (function block 001, B001) to hold the pump index, the VM (Variable Memory) parameter mapping to expose the counter value to the network analog input domain, a "Network Analog Input" block (AI) to read the value back into the logic, and ten "Analog Threshold Triggers" (function block 003, B003) to decode the index into discrete outputs Q1..Q10.

The pattern matters for two practical reasons. First, on LOGO! 0BA8 and later, the counter's "Ref" output is typed and cannot be wired directly to a free-floating comparator chain without an intermediate marker; VM mapping is the cleanest way to publish the value for both the local logic and any S7 client (e.g., TIA Portal HMI). Second, the threshold-trigger approach gives a 1-count hysteresis band around each pump number, eliminating the chatter that direct equality comparators produce during key-hold transitions. The full VM-mediated design is documented in the LOGO! 8 system manual and the LOGO!Soft Comfort V8.4 manual.

Prerequisites and Hardware Sizing

• LOGO! 8 BM Basic Module: catalog 6ED1052-1xx08-0BAx (0BA8 family). Recommended firmware FS04 (LOGOSYS V8.3) or FS05 (LOGOSYS V8.4) for stable VM behavior. See the LOGO! 8 system manual, chapter "Technical data".
• LOGO! TDE text display: catalog 6ED1055-4MH08-0BA0 (FS04) or 6ED1055-4MH08-0BA1 (FS05). The TDE provides the F1, F2, ESC, OK, and four cursor keys that this design binds to digital inputs I1..I5.
• LOGO!Soft Comfort V8.3 or V8.4: catalog 6ED1058-0BA08-0YA1 (V8.3) or 6ED1058-0BA08-0YA2 (V8.4). Online test, parameter VM mapping view, and Watch Table require V8.2 or later.
• Power supply sizing: a 24RCE (6ED1052-1MD08-0BAx) draws 0.7 W idle and 1.7 W with all relays energized. The internal 24 V sensor bus delivers up to 4 A. For 115-240 V supply, use a 230RCE (6ED1052-1FB08-0BAx). Reference: LOGO! 8 system manual.
• Output count check: a base 0BA8 BM exposes Q1..Q4. Adding one AM2 (6ED1055-1MB00-0BA2) extends the digital rail to Q1..Q8; a second AM2 extends to Q1..Q10. Plan the AM2 count before writing the program; LOGO!Soft Comfort only enables Q1..Q8 by default on a single-AM2 setup and will not permit you to drag Q9/Q10 onto the diagram until the second AM2 is in the project setup.

Block Function Reference

VM Memory and Network Analog I/O Architecture

LOGO! 8 reserves a 16-bit address space known as VM (Variable Memory) for parameter mapping, inter-block data exchange, and S7-style network analog I/O. The relevant ranges per the system manual are:

Counter B001's "Ref" output (the current integer count) can be parameterized to a VM address by clicking the Ref pin in LOGO!Soft Comfort and selecting "VM address" in the assignment dropdown. Mapping the Ref to VD0 (dword 0) places the live count into the lower 16 bits of the dword, visible as VW0. The high word of VD0 is also a valid 16-bit value, so the community-recommended approach is to read VW2 with a network analog input - this places the index into a separate analog variable that does not overlap with any other 16-bit VM that the lower dword shares. Equivalent configuration: AI Type="Word", Source=VD0 with Gain=1; both reads work.

Architecture Diagram

Step-by-Step Implementation

Step 1 - Build the Counter

1. Open LOGO!Soft Comfort and create a new project. Select the target BM (e.g., 0BA8 / 0BA9) and TDE in the project setup wizard. The wizard sets the maximum block count and the available VM range.
2. Drag an Up/Down Counter (B001) onto the diagram. In the block properties, set:
   • Cnt input: digital input I3 (cursor right = count up)
   • Dir input: digital input I4 (cursor left = count down) - tie low if not used
   • On threshold: 10
   • Off threshold: 0
   • Start value (preset, optional): 0
3. Click the Ref pin. In the assignment dropdown, choose "VM address" and enter VD0. The Ref pin is now the live 0..10 count, written to VM dword 0.
4. Optional: in the Remanence tab, enable the retentive flag if you need the selected pump to survive a power cycle. The 0BA8 retains 30 bytes of application data (see system manual "Remanence" chapter).

Step 2 - Read the Value via Network Analog Input

1. Drag a Network Analog Input (AI) block onto the diagram. In LOGO!Soft Comfort V8.3/V8.4 the block is in the analog library under "Network".
2. Open AI properties. Set Type="Word" and Source=VW2. The AI block now reports the counter value as an analog quantity 0..10 on its output pin AQ.
3. Wire the AQ pin to a marker block (M1) so other parts of the program can read the "selected pump index" without re-entering the analog chain. M1 is also the tag exposed to a TIA Portal HMI if you are using LOGO! as an S7 client.

Step 3 - Add 10 Analog Threshold Triggers

1. Drag ten Analog Threshold Trigger blocks (B003-a through B003-j) onto the diagram. Each one will decode a single integer value of the index into one digital output.
2. Configure each block identically except for the threshold values:

   Block	On threshold (>=)	Off threshold (<)	Output
   B003-a	1	2	Q1 (pump 1)
   B003-b	2	3	Q2 (pump 2)
   B003-c	3	4	Q3 (pump 3)
   B003-d	4	5	Q4 (pump 4)
   B003-e	5	6	Q5 (pump 5)
   B003-f	6	7	Q6 (pump 6)
   B003-g	7	8	Q7 (pump 7)
   B003-h	8	9	Q8 (pump 8)
   B003-i	9	10	Q9 (pump 9)
   B003-j	10	11	Q10 (pump 10)

3. Set the analog input Ax source of every B003 block to the same marker (M1) that holds the AI output. Verify Gain=1.0 and Offset=0 on every block.

Step 4 - Wire Outputs Q1..Q10

Connect the output of B003-a to Q1, B003-b to Q2, and so on. The Q pins must be assigned on a LOGO! BM that physically exposes them. On a 0BA8 base module Q1..Q4 are onboard; Q5..Q8 require one AM2 extension, and Q9..Q10 require a second AM2 extension module. See the LOGO! 8 system manual, chapter "Maximum setup".

Step 5 - Tie the TDE Keys to Inputs and Configure the Menu

1. Open Tools > Display > Key Assignment in LOGO!Soft Comfort.
2. Map the TDE keys as follows:

   TDE key	Drives digital input	Function
   F1	I1	Open menu / latches M2
   F2	I2	Confirm run / latches M3 (run command)
   Right	I3	Counter increment (B001 Cnt)
   Left	I4	Counter decrement (B001 Dir)
   ESC	I5	Cancel menu / reset M2

3. Configure two message texts on the TDE:
   • Idle: "Pump 1..10 - F1 menu". Visible when M2 is low.
   • Menu: "Pump N (current M1 value) - F2 start". Visible when M2 is high. The M1 marker is inserted as a placeholder token.
4. Add an On-Delay (B004) of 100 ms between the TDE keys and the B001 input to debounce contact bounce. The 0BA8 default scan time is ~10 ms, but a single debounce step is recommended for operator panels.

State Machine View

Threshold Trigger Configuration Reference

Parameter Reference Table

Verification Procedure

1. In LOGO!Soft Comfort, click Tools > Online Test (or press F11). The project must already be transferred to the BM via Ethernet.
2. Open the Watch Table (View > Watch Table). Add the following tags: B001/CV (current value), VD0, VW2, M1, Q1..Q10.
3. Press F1 on the TDE. Confirm M2 latches high and the message text "Select pump 1..10" appears on the TDE.
4. Press cursor right three times. Confirm B001/CV = 3, VW2 = 3, M1 = 3, and Q3 lights. Q1 and Q2 must remain low.
5. Press cursor right once more. Confirm B001/CV = 4, Q3 drops, Q4 lights. The 1-count hysteresis should keep Q3 from flickering at the boundary.
6. Press F2. Confirm M3 latches and the run command is asserted on the field wiring side (pump contactor pulls in, current monitor on Q3 or Q4 shows load).
7. Hold cursor right past 10. Confirm B001 saturates at 10 and does not roll to 0 (this is the threshold-On behavior of B001 with On=10).
8. Power-cycle the BM. Confirm B001 resets to 0 (no retentive flag set) and Q1..Q10 all drop. The default is non-retentive; set the retentive flag on B001 if you need power-loss memory.
9. Inspect VM 0 in LOGO!Soft Comfort parameter view (View > Parameter VM Mapping) and confirm VD0 reflects the live counter value. The expected pattern: 0x0000 0000 at idle, 0x0000 0004 at index 4, etc.

Troubleshooting Matrix

Alternative Implementations

Direct Comparator Chain (B001 + 10 Comparators)

Where the project has spare comparator blocks and you want to avoid VM mapping, a chain of "Analog Comparator" blocks (function block 002, B002) can read B001's Ref pin directly. Each B002 has Ax = B001/Ref and Ay = constant 1..10. This is the approach some LOGO!Soft Comfort versions recommend when VM mapping is unavailable, but it costs ten B002 blocks plus the B001, and the Ref pin must be wired to all of them - acceptable on 0BA7 and earlier where the Ref pin is an untyped analog output, but visually cluttered in larger programs and not viable on 0BA8 where the typed Ref pin cannot be wired to ten blocks at once without an intermediate marker.

Shift Register Cascade (Single-Pump Mode)

For installations where only one pump may run at a time, a shift register (B004 "Shift Register" or a string of latching relays driven by a single "edge" input) gives mutual exclusion for free. However, this loses the "select a number, confirm with F2" UX of the original design and is a different functional requirement. Choose this only if the system spec is "rotate through the 10 pumps on each F2 press".

LOGO! 8 with S7-1200 / TIA Portal HMI

On a LOGO! 8 BM with a S7 connection, expose the VM area as a DB block in TIA Portal. The M1 marker becomes a tag that any Comfort Panel / KTP can read and display. Threshold trigger output bits can also be mapped to the BM's own Q area for direct feedback in the TDE message text. Reference: LOGO! 8 system manual, chapter "LOGO! as S7 client". For systems that outgrow LOGO! 8 (typically 200+ blocks or 16+ pumps), migrate to a SIMATIC S7-1200 (CPU 1214C DC/DC/DC, catalog 6ES7214-1AG40-0XB0) with the same S7 source from TIA Portal - the threshold-trigger pattern is replaced by an FC that compares an integer tag to constants and sets output bits directly.

LOGO! 8 with DM16 Expansion

If you need more than 10 outputs and cannot add AM2 modules, use a DM16 (6ED1055-1CB00-0BA0) digital module. It adds 8 digital inputs and 8 digital outputs in the same housing, but only on a S7-1200 / ET 200SP, not on a LOGO! 8 BM. For a LOGO! 8 with more than 10 Q outputs, the practical limit is 24 Q via three AM2 modules (Q1..Q24, with the BM using the first 8).

Modbus TCP Exposure of VM

LOGO! 8 firmware FS04 and later supports Modbus TCP server on port 502. The VM area is exposed as holding registers at offset 0x0000 - 0x03FF. Read VW2 at register address 0x0002 (word). This is the cleanest way to publish the selected pump index to a SCADA system without TIA Portal. See the LOGO! 8 system manual chapter "Modbus communication".

Field Commissioning and Safety Notes

• Relay derating: a 0BA8 relay output is rated 8 A resistive / 2 A inductive (AC-15) per the system manual. For three-phase pump contactors driven by Q1..Q10, route each Q through an interposing relay rated for the contactor coil. The derated cycle life at full load is approximately 100,000 operations; for a 1 start/hour application this gives ~11 years of life before contact replacement.
• Power-budget check: ten simultaneously-energized relay coils on the LOGO! outputs draw approximately 10 x 0.5 W = 5 W of contact dissipation, plus the contactor coil current. The 24RCE sensor bus is rated 4 A; if you power the contactor coils from this rail, leave 1 A headroom for sensor supply.
• Retentive counter flag: enabling the retentive flag on B001 reserves 4 bytes of remanent memory. The 0BA8 has 30 bytes of remanent memory; ten retentive counters would exhaust it, leaving no space for the retentive run command marker. Plan retention against the 30-byte budget.
• Phase-failure upstream: a LOGO! BM cannot detect loss of any phase on its 230 V supply. Place a phase-failure / phase-sequence relay (e.g., Siemens 3UG4511) upstream of the pump bank. Wire its output to an enable input on the run command, so a missing phase blocks all 10 starts.
• Debounce TDE keys: the 0BA8 default scan time is ~10 ms, but the TDE F1/F2 keys have a 30 ms mechanical bounce. Add an On-Delay (B004) of 100 ms between TDE keys and the B001 input to debounce.
• EMC: route the TDE cable away from VFD power cables (minimum 200 mm parallel clearance). The TDE cable carries logic-level signals and is susceptible to common-mode noise from variable-frequency drives. Use shielded cable for runs longer than 3 m.
• Compliance: program structure follows IEC 61131-3; the operator panel wiring follows IEC 60204-1 (emergency stop category 0 or 1, 24 V control). Reference: IEC 61131-3:2013, IEC 60204-1:2016. For SIL applications, the LOGO! 8 is not rated above SIL 1; a SIMATIC S7-1200F or S7-1500F is required for SIL 2/3.

Relay Derating Formula

For ten 24 V DC pump contactor coils driven by the LOGO! Q outputs, the per-contact resistive load and inductive derating is:

P_coil = V_coil x I_coil (W per coil)

For a 24 V DC / 5 W contactor coil:

I_coil = P_coil / V_coil = 5 W / 24 V = 0.208 A (208 mA)

Ten coils summed: 10 x 0.208 A = 2.08 A peak. The 24RCE sensor bus is rated 4 A, so 2.08 A leaves 1.92 A for sensors. If sensor current exceeds 1.92 A, power the contactor coils from a separate 24 V DC power supply and use the LOGO! Q outputs as dry contacts only.

Expected mechanical life (per system manual): 10 million operations no-load; 100,000 operations at 8 A resistive, 240 V AC. The cycle-life ratio at typical 1 start/hour and a 50% duty cycle is:

Lifetime_years = 100000 / (1 start/hour x 8760 hours/year) = 11.4 years

Plan contactor-coil replacement on a 10-year preventive-maintenance cycle, or upgrade to a solid-state AM2 AQ module (6ED1055-1MA00-0BA2) for unlimited cycles.

Frequently Asked Questions