Linking Siemens LOGO! Controllers: ASI Bus, Ethernet, and Field-Proven Workarounds

The Siemens LOGO! is positioned as a "smart relay" or logic module rather than a fully networked PLC. Although the platform exposes a programming port, an optional AS-Interface (ASI) slave module, and (on LOGO! 8 hardware) an Ethernet interface, direct peer-to-peer communication between two LOGO! controllers is not a documented feature of the platform. This reference consolidates the engineering options for getting data from one LOGO! to another, the electrical hazards of the programming port, and the practical limits of the bit-wiggle workaround commonly deployed in the field.

1. Why LOGO!-to-LOGO! Linking Is Non-Trivial

LOGO! was designed as a compact replacement for a few relays and timers, not as a peer in a distributed control network. The native I/O bus behind a LOGO! base module is proprietary and supports only LOGO! expansion modules (DM8/DM16, AM2/AM2 RTD, etc.) connected directly to the right-hand bus connector. There is no bus arbiter, no master slot, and no documented slave-to-slave protocol. As a result, engineers who need data to flow between two LOGO! devices must use one of three indirect methods:

1. AS-Interface (ASI) bus with an external master (typically an S7-200 with CP 243-2 or a third-party ASI master gateway).
2. Ethernet network variables on LOGO! 8 (6ED1052-*) hardware, which supports up to 16 LOGO! clients on a single Ethernet subnet.
3. Bit-wiggle I/O between two transistor outputs of one LOGO! and two discrete inputs of the other, clocked manually from the first device's program.

Each method has different bandwidth, latency, wiring, and electrical-safety characteristics. The remainder of this article describes each one in detail, including the parts catalog numbers, ASI slave register maps, and the S7-200 master configuration required to bridge them.

2. LOGO! Hardware Architecture and Communication Interfaces

LOGO! generations relevant to multi-device communication:

The integrated Ethernet port on LOGO! 8 is the only platform variant that ships with native peer-to-peer variable exchange. Older 0BA6/0BA7 hardware cannot be retrofitted with Ethernet; the only communications option remains the ASI slave module.

3. AS-Interface: Master-Slave Topology for LOGO!

The LOGO! CM AS-i (catalog 6ED1055-1MM00-0BA2) is an ASI slave only. It does not arbitrate the bus, does not power other slaves, and cannot be daisy-chained to a second LOGO! ASI module for direct exchange. To move a flag bit from LOGO! A to LOGO! B, the data must travel: LOGO! A → ASI slave → ASI cable → ASI master → master PLC program → master ASI write → ASI cable → LOGO! B ASI slave.

3.1 ASI Bus Electrical Parameters (IEC 62026-2 / EN 50295)

3.2 LOGO! ASI Slave Data Image

The LOGO! CM AS-i exposes four digital inputs (DI0–DI3) and four digital outputs (DO0–DO3) on the ASI slave address. From the LOGO! program, the inputs arrive in flags I16–I19 (or network inputs NI1–NI4 in the LOGO! Soft Comfort project) and the outputs are written through Q16–Q19 (or network outputs NQ1–NQ4).

Each LOGO! CM AS-i is a single standard slave. To pass one Boolean from LOGO! A to LOGO! B, you need: 1 bit out of A's four DO slots, master program to copy it, and 1 bit into B's four DI slots. With four bits per device you can transfer up to four flags per direction using two slaves and a master.

4. Using an S7-200 as ASI Master

The Siemens S7-200 CPU (any 22x family) can host a CP 243-2 ASI master module (catalog 6GK7243-2AX01-0XA0) on its right-side I/O bus. With STEP 7 Micro/WIN and the optional ASI library, the master can read DO0–DO3 from LOGO! A's slave, store the bit in an internal marker (M area), and write it to DI0–DI3 of LOGO! B's slave on the next ASI cycle.

4.1 Wiring the Master Segment

1. Mount the CP 243-2 directly to the S7-200 CPU (no slotting flexibility; the CP occupies the first position to the right of the CPU).
2. Wire the ASI cable from the CP 243-2 ASI terminals to a Siemens ASI power supply (e.g., 3RX9501-0BA00 or 6EP3333-8SB00-0AY0) using the yellow ASI profile cable.
3. Connect LOGO! A's CM AS-i to the same segment at slave address 1; LOGO! B's CM AS-i to slave address 2.
4. Configure both LOGO! CM AS-i modules to A/B addresses only if you are using an extended ASI master (CP 243-2 supports up to 62 slaves in A/B mode, doubling the per-segment count).

4.2 S7-200 Program Skeleton (ST, Micro/WIN)

// Read DO0 from LOGO! A (slave 1) and pass it to DI0 of LOGO! B (slave 2).
// Requires the USS/ASI library (SBR 14 ASI x Execute / x Read) configured
// in Micro/WIN Tools → Instruction Wizard → ASI.

VAR
  Slave1Read : BYTE;   // Bit pattern of slave 1 outputs (read from master)
  Slave2Write: BYTE;   // Bit pattern to write to slave 2 inputs
END_VAR

// Cycle 1: read slave 1
ASI_RD_SL(Slave := 1, Code := 16#70, Index := 0,
          Data  := Slave1Read, Done => bDone1);

// Capture DO0 of LOGO! A into M0.0
M0.0 := Slave1Read.0;

// Cycle 2: write M0.0 to DI0 of slave 2
Slave2Write.0 := M0.0;
ASI_WR_SL(Slave := 2, Code := 16#70, Index := 0,
          Data  := Slave2Write, Done => bDone2);

Throughput in this configuration is bounded by the ASI cycle time of the CP 243-2 (≤5 ms for 31 slaves, far less for two). End-to-end latency from LOGO! A flag change to LOGO! B input recognition is typically 10–25 ms.

5. LOGO! 8 Ethernet Network Variables

LOGO! 8 (6ED1052-*08-0BA0) introduces native Ethernet networking. Each LOGO! 8 can publish a block of network variables to the LAN, and any other LOGO! 8 on the same subnet (up to 16 remote LOGO! peers) can subscribe to them. This is the only Siemens-supported peer-to-peer mechanism between LOGO! devices.

5.1 Configuration in LOGO! Soft Comfort

1. Open the project and select Tools → Ethernet Connections.
2. Add an S7 connection or LOGO!-to-LOGO! connection (the latter is LOGO!-native and does not require an S7 CPU).
3. On the server LOGO!, define a network variable table (up to 64 network inputs and 64 network outputs total across the project).
4. On the client LOGO!, map those variables to local inputs/outputs (NI1..NI64 and NQ1..NQ64) for use in the FBD/LD program.
5. Assign static IPs or enable DHCP on each device; ensure the subnet mask and gateway allow direct L2/L3 reachability.

5.2 Throughput and Limits

For 4-bit flag exchange, LOGO! 8 Ethernet replaces the entire ASI + S7-200 stack with a single crossover cable and a few clicks in LOGO! Soft Comfort. Older 0BA6/0BA7 hardware cannot be upgraded in the field to gain Ethernet; it must be replaced.

6. Programming Port Hazards on AC-Powered LOGO!

The RJ11 programming port on 0BA6/0BA7 hardware shares the same 0 V reference as the LOGO! internal logic supply. There is no galvanic isolation between the port's signal ground and the LOGO! power input ground. On a 24 V DC-powered LOGO! this is benign; on a 115/230 V AC-powered LOGO! the situation is different.

• Different phases / different transformers: If LOGO! A is fed from phase L1 and LOGO! B from phase L2 of the same panel, the programming-port grounds sit at different potentials. Connecting the two ports ties the neutrals together through the programming cable and can produce fault currents through unintended paths.
• Different panels / different buildings: The neutral-to-ground bond can differ between panels. Even short programming cables can carry nuisance currents that interfere with the serial link or, in the worst case, present a touch-voltage hazard to the operator's laptop.
• Long runs: Field reports describe reliable operation only across distances where the two devices share a common low-impedance ground reference. Anything beyond a few metres should be treated as suspect.

7. Bit-Wiggle Workaround: Two-Wire, ~8 Bits/Second

Where ASI hardware or Ethernet is unavailable, a deterministic bit-banged link can be implemented between two LOGO! devices using one digital output as clock and one as data on the transmitter side, plus two digital inputs on the receiver side. Field deployments report reliable operation at 8 bits/second using transistor outputs.

7.1 Wiring Diagram

Both LOGO! devices must share the same 24 V DC supply reference (a common M terminal). Do not attempt this across galvanically isolated supplies or AC-powered LOGO! units.

7.2 Transmitter Logic (LOGO! Soft Comfort FBD pseudocode)

// On rising edge of B00 (send trigger), shift out 8 bits of
// flag word VW0 from bit 0 to bit 7 on Q1, clocked on Q2.
// Cycle period: 125 ms per bit => 1 second per byte (8 bits/sec).

B01 = B00 (one-shot, edge-triggered via "AND with NOT B01")
B02 = B01 AND NOT B07        // shift counter enable
B03 = T1 (pulse timer, 60 ms on / 65 ms off)
Q2  = B03                     // clock line toggles every cycle
Q1  = B03 AND VW0.bit   // data line follows current bit while clock = 1
B07 = (counter of B03 edges) == 8

7.3 Receiver Logic

// On each rising edge of I2 (clock), latch the current value of I1
// into the next position of an 8-bit shift register built from
// eight flag markers M0.0..M0.7. After 8 edges, copy the byte
// into the target flag word and reset the counter.

M0.0 = I2 AND NOT M_edge_latch_old
For n = 1..7:
  M0.n = M0.(n-1) on rising edge of I2   // shift chain
Counter = count of I2 rising edges
When Counter = 8:  target_VW = M0.7..M0.0, reset Counter

7.4 Practical Limits

The bit-wiggle method is appropriate for slow status mirroring (run/stop, fault, level reached, count complete). It is not a substitute for ASI or Ethernet when higher throughput, deterministic latency, or galvanic isolation is required.

8. Alternative Controllers with Native Multi-Device Networking

Engineers who need more than what LOGO! provides without bolting on an S7-200 master should evaluate the following compact PLC/logic-module families, each of which supports native peer-to-peer or master-master networking out of the box.

For new designs requiring more than four flags across two or more logic modules, the easyE4 (Eaton) and Modicon M221 (Schneider) are credible direct replacements that add native Ethernet-based multi-device networking at a small price premium.

9. Comparison Matrix: Which Method to Choose

10. Verification Procedures

After commissioning any of the three methods, perform the following checks before releasing the system to production.

10.1 ASI Verification (with S7-200 Master)

1. Open STEP 7 Micro/WIN and confirm both LOGO! CM AS-i slaves appear in the ASI status table with the expected slave addresses.
2. Force a flag high in LOGO! A and watch the corresponding M-bit in the S7-200 status chart toggle within one ASI cycle (≤5 ms plus master scan).
3. Force the same M-bit in the S7-200 and confirm LOGO! B's network input indicator changes within one additional ASI cycle.
4. Pull the ASI cable between CP 243-2 and slave 1; the S7-200 should raise an ASI fault (SF LED on CP 243-2) and slave 1 should be reported as failed.

10.2 LOGO! 8 Ethernet Verification

1. From LOGO! Soft Comfort, use Tools → Ethernet Connections → Diagnostics to confirm both peers report OK.
2. Toggle a flag on the server LOGO! and observe the subscribed network input on the client LOGO! in the online monitor.
3. Disconnect the Ethernet cable; the client should report connection loss within the configured timeout.

10.3 Bit-Wiggle Verification

1. Send the byte pattern 0xAA (alternating bits) repeatedly and confirm the receiver shift register reproduces it without drift.
2. Inject a 100 ms brown-out on the transmitter's 24 V supply and verify the receiver does not latch a corrupted byte (or that the application rejects bytes that fail a parity / start-stop check).
3. Measure the actual bit period with a scope on the clock line; confirm it matches the 125 ms design value ±10%.

11. Frequently Asked Questions