Problem Definition
Siemens LOGO! 8 logic modules in the 24 V DC transistor-output variant advertise their four digital outputs as 24 V DC, 0.3 A, short-circuit-proof, and overload-protected. To a controls engineer familiar with discrete transistors, a "24 V, 0.3 A" output suggests a generic low-side or push-pull driver that can both source and sink. To a panel builder wiring a string of common-anode LED indicators, it suggests that pulling a Q terminal "low" will return current from the LED to M (ground). Both assumptions are wrong, and the source of the confusion is the silicon part that actually drives the Q terminal: the Infineon ITS4140N high-side switch from the PROFET 12 V family. The ITS4140N can only raise the Q terminal to +24 V; it cannot pull it to 0 V, and its "off" state is a high-impedance node whose actual voltage is set by leakage paths and the load. The remainder of this article explains the silicon-level reason for the limitation, quantifies the off-state terminal voltage, and provides four engineering workarounds that preserve the original panel design without violating the ITS4140N electrical constraints.
The failure mode is unambiguous: a common-anode LED with the cathode returned to a LOGO! Q terminal either stays dark or glows at a small fraction of nominal brightness when the Q output is commanded off, because the cathode does not return to 0 V. The LED sees only the difference between the 24 V anode rail and the floating cathode potential, which usually lands at 22 V or higher once the LED forward drop is subtracted. The few hundred microamps of leakage through the LED at that voltage are within the ITS4140N off-state drain-leakage limit, so the part is operating inside its data sheet envelope; the design, not the silicon, is at fault.
LOGO! 8 24CEoT). Modules with relay outputs (the "R" variants) provide a true potential-free changeover contact and can source or sink within the published contact rating; they are not subject to the ITS4140N limitation.LOGO! 8 Transistor Output Architecture
Each of the four Q outputs on a 24 V DC LOGO! 8 transistor module is wired to a single channel of the Infineon ITS4140N inside the module sealed housing. The ITS4140N is a single-channel high-side switch: it contains a vertical N-channel power MOSFET whose drain is brought out to the Q terminal and whose source is bonded internally to the module +24 V supply (the L+ terminal). When the LOGO! logic commands the output on, an internal charge pump generates a gate voltage about 10 V above the source, fully enhancing the N-channel MOSFET and connecting the drain to the +24 V rail through a typical on-state resistance of 140 mΩ. When the output is commanded off, the gate is pulled to the source, the channel pinches off, and the drain is left high-impedance.
Three electrical consequences follow directly from this single-channel high-side topology and are the root cause of every sinking-related issue observed in the field:
- Source-only conduction. The output can only raise a load toward +24 V. It cannot pull a load toward 0 V. Any wiring that requires the LOGO! to act as a low-side return to M is electrically incompatible with the silicon.
- Undefined off-state terminal voltage. The "off" Q terminal is a high-impedance node, not a 0 V rail. The voltage that appears at the terminal is set by the attached load, by the ITS4140N drain-to-source body diode, and by module-internal ESD and bias networks. A floating terminal may measure anywhere from 0 V to one diode drop below +24 V on a high-impedance voltmeter.
- Source-only current limit. The ITS4140N 2 A (min) / 5 A (max) active current limit is enforced by the on-chip protection logic and thermal shutdown. The part is designed to source 0.3 A to a load connected between Q and M; it does not enforce a sink-current limit because no sink path exists. A short from Q to M is handled gracefully (thermal cutoff at roughly 150 °C junction), but the part cannot deliver 0.3 A into a short to L+ (because that is the source path, not the load path).
These properties are not anomalies; they are the published behavior of every member of the Infineon PROFET 12 V family, of which the ITS4140N is a representative single-channel example. The block diagram from the ITS4140N data sheet shows the drain terminal D brought out to the application while the source terminal S is bonded to Vbb internally. A simplified reproduction appears below.
Inside the ITS4140N: Why It Cannot Sink
The ITS4140N combines a vertical N-channel DMOS output stage, a charge pump, gate driver, current-sense front end, and integrated protection on a single die. The most important electrical parameters from the published data sheet are summarized in the table below; refer to the live Infineon ITS4140N product page for the latest revision.
| Parameter | Symbol | Min | Typ | Max | Unit |
|---|---|---|---|---|---|
| Operating supply voltage (Vbb) | Vbb | 4.5 | — | 42 | V |
| On-state resistance, Tj = 25 °C | RDson | — | 140 | 200 | mΩ |
| On-state resistance, Tj = 150 °C | RDson | — | — | 320 | mΩ |
| Nominal load current | IL(nom) | — | 1.4 | — | A |
| Active current limit | IL(LIM) | 2.0 | — | 5.0 | A |
| Off-state drain leakage | IDL | — | — | 5 | µA |
| Standby current, logic = 0 | Ibb(off) | — | — | 25 | µA |
| Thermal shutdown | Tj(SD) | — | 150 | — | °C |
| Status output | ST | Open-drain to GND, low = fault | |||
The charge pump is essential: an N-channel MOSFET used as a high-side switch needs Vgs greater than the threshold to fully turn on, and the source is already at +24 V. The pump generates a gate voltage roughly 10 V above the source, allowing full enhancement. When the output is commanded off, the gate is shorted to the source and the channel is firmly pinched off. The output is then truly high-impedance, but two parasitic paths still affect the terminal voltage: the drain-to-source body diode and the off-state drain leakage current IDL.
Voltage Behavior of an Inactive Q Output
With the Q output commanded off, the ITS4140N drain is nominally high-impedance, but three mechanisms combine to define the actual terminal voltage observed at the LOGO! screw terminal:
- Body-diode clamp. If the load forces the drain above Vbb, the body diode conducts and clamps the terminal to Vbb + 0.7 V. For the LED scenario with anode at 24 V, the LED itself becomes the clamping element; the LED forward drop of about 2 V appears in series with the leakage path and the ITS4140N own drain capacitance.
- Off-state drain leakage. The data sheet specifies IDL of a few microamperes at 25 °C. This current, multiplied by any external resistance to a defined potential, develops a voltage across the Q terminal. A 100 kΩ pull-down would let the leakage develop 0.5 V at most; a 10 MΩ probe would let it develop the full 24 V.
- Module-internal bias. The LOGO! schematic includes ESD protection diodes to both L+ and M, plus a status-feedback network that pulls the terminal weakly toward L+. The combined effect is that an unconnected Q terminal measures 20–24 V on a 10 MΩ digital multimeter.
For the common-anode LED application, the practical effect is that the LED cathode is held at Vbb − Vf(LED) − Ileak · Rp, where Rp is any pull-down the panel builder installed. With no pull-down (Rp = ∞), the cathode sits at Vbb − Vf ≈ 22 V; the LED sees only Vf, which is at or below the conduction knee. The LED either stays dark or flickers dimly. With a 10 kΩ pull-down, the leakage current develops only 0.05 V across Rp and the cathode sits very close to 0 V, lighting the LED at full brightness. The following table quantifies the relationship.
| Pull-down resistance (Q to M) | Off-state Q voltage | LED anode-cathode voltage | LED brightness |
|---|---|---|---|
| None (open circuit) | 22.0 V (clamped by LED) | 2.0 V (at Vf knee) | Off or very dim |
| 1 MΩ | 18.0 V (5 µA × 1 MΩ) | 6.0 V (above knee) | Moderate, unstable |
| 100 kΩ | 5.5 V (clamping begins to give way) | 18.5 V | Bright |
| 10 kΩ | 0.5 V | 23.5 V (limited by series R) | Full (set by Rs) |
| 1 kΩ | 0.05 V | 23.95 V (high waste current) | Full (set by Rs) |
The 10 kΩ to 100 kΩ range is the practical sweet spot: low enough to develop negligible voltage from leakage, high enough to limit the waste current when the Q output is on.
Why the Common-Anode LED Wiring Fails
The intended panel design is a single +24 V rail feeding every LED anode in parallel, with each LED cathode returning to a dedicated LOGO! Q output. The panel builder mental model is "Q on = 24 V at cathode = LED off; Q off = 0 V at cathode = LED on." That mental model assumes the LOGO! is a low-side switch. It is not. The actual behavior is summarized in the comparison table below.
| LOGO! state | Q terminal voltage | LED current | Result | Matches intent? |
|---|---|---|---|---|
| Q1 ON, no pull-down | ≈ 24 V (RDson · IL ≈ 0.1 V at 10 mA) | 0 mA | LED off | Yes |
| Q1 OFF, no pull-down | ≈ 22 V (clamped by LED) | ≈ 0.5 mA leakage | LED off or very dim | No |
| Q1 ON, 10 kΩ pull-down | ≈ 24 V (pulled to Vbb by MOSFET) | 0 mA | LED off | Yes |
| Q1 OFF, 10 kΩ pull-down | ≈ 0.05 V | ≈ 10 mA (set by Rs) | LED on at full brightness | Yes |
The middle row is the failure mode. The correct mental model is that the LOGO! Q output is a switch between Q and +24 V, not between Q and 0 V. Once that model is internalized, the workaround options become obvious.
Workaround A — Use a Relay Output Module
If the application tolerates a relay, the simplest fix is to swap the LOGO! base module (or add a digital output expansion module) for the equivalent relay variant. Common catalog numbers in the LOGO! 8 family (verify exact suffixes with the Siemens product configurator) include:
-
6ED1052-1MD08-0BA1— LOGO! 8 24CEoT, 8 DI / 4 DO transistor (the starting point) -
6ED1052-1HB08-0BA0— LOGO! 8 24CEoR, 8 DI / 4 DO relay, 5 A contacts -
6ED1052-1FB00-0BA1— LOGO! 8 230RCo, 8 DI / 4 DO relay, 8 A contacts -
6ED1055-1HB00-0BA1— LOGO! DM8 24R expansion, 4 DO relay -
6ED1055-1NB10-0BA1— LOGO! DM16 24R expansion, 8 DO relay
Relay outputs present a potential-free changeover contact, so each Q terminal can source or sink within the published contact rating. For a 10 mA LED string the contact rating is not a concern. The drawbacks are mechanical wear and slower switching time. The LOGO! 8 relay is rated for 100 000 cycles at full load (5 A, 250 V AC) and 1 000 000 cycles at reduced load (0.1 A). The mechanical operate time is about 10 ms, which is acceptable for status indicators and slow control but unsuitable for pulse-width modulation or fast safety functions.
6ED1052-1MD08-0BA1) can drive relay expansion modules (DM8 24R) over the internal bus. The relay outputs on the expansion are independent of the base module outputs and can sink or source as needed. A mixed I/O configuration is fully supported by the LOGO! Soft Comfort engineering tool from firmware version 8.0 onward.Workaround B — External Pull-Down Resistor
When a relay is unacceptable — for example, when the outputs must switch at high cycle rates, or the panel must remain all-electronic for reliability reasons — an external pull-down resistor between the Q terminal and the M terminal forces the inactive output to a defined 0 V. The size of the resistor is a trade-off between LED brightness, waste current, and the LOGO! 0.3 A per-output budget.
Worked example: a single 24 V red indicator LED with Vf = 2.0 V and target IF = 10 mA.
- Series current-limiting resistor:
Rs = (Vsupply − Vf) / IF = (24 − 2) / 0.010 = 2 200 Ω. Use a 2 200 Ω E12 part rated for 1/4 W or more. - Pull-down resistor: Rp must allow 10 mA of LED current to flow when Q is off. Setting Rp = 2 200 Ω gives a Thevenin resistance of Rs || Rp = 1 100 Ω, so the LED current drops to about 22 V / 3 300 Ω = 6.7 mA. The LED will be visibly dimmer but still well above the 5 mA threshold for most indicator LEDs.
- Waste current when Q is on: 24 V / 2 200 Ω = 10.9 mA. This is small relative to the 300 mA output rating, but it is dissipated continuously whenever the LED is supposed to be off.
- Power in pull-down: 24 V × 10.9 mA = 262 mW. Use a 1/2 W part to keep the resistor cool, or a 1 W part if the panel ambient is above 50 °C.
For higher-brightness LEDs (white, blue, green) the Vf of 3.0–3.4 V changes the math. With Vf = 3.2 V and IF = 20 mA, Rs = (24 − 3.2) / 0.020 = 1 040 Ω (use 1 000 Ω E12). The same Rp = 1 000 Ω limits the on-state waste to 24 mA, which dissipates 576 mW in the pull-down — too much for a 1/4 W part. Choose a 1 W resistor, or reduce the LED current to 10 mA and use 2 000 Ω for both Rs and Rp.
| LED color (typ Vf) | Target IF | Rs | Rp | Waste (Q on) | Rp power (Q on) | Min Rp rating |
|---|---|---|---|---|---|---|
| Red 624 nm (2.0 V) | 10 mA | 2 200 Ω | 2 200 Ω | 10.9 mA | 262 mW | 1/2 W |
| Yellow 590 nm (2.1 V) | 10 mA | 2 200 Ω | 2 200 Ω | 10.9 mA | 262 mW | 1/2 W |
| Green 525 nm (3.2 V) | 10 mA | 2 000 Ω | 2 000 Ω | 12.0 mA | 288 mW | 1/2 W |
| Blue 470 nm (3.2 V) | 10 mA | 2 000 Ω | 2 000 Ω | 12.0 mA | 288 mW | 1/2 W |
| White 6500 K (3.2 V) | 15 mA | 1 500 Ω | 1 500 Ω | 16.0 mA | 384 mW | 1 W |
The pull-down approach has three operational caveats. First, the waste current is drawn from the LOGO! output and counts against the 0.3 A per-output budget; four LEDs with 11 mA waste each total 44 mA, still well within the 300 mA limit but worth tracking on the panel schematic. Second, the Q terminal briefly passes through its active region during every state transition; the inrush current equals the steady-state waste current and does not trigger the ITS4140N 2 A current limit. Third, the pull-down path defeats the diagnostic value of the open-drain status pin on the ITS4140N, because the status pin monitors drain current which now always includes the pull-down contribution.
Workaround C — External P-Channel MOSFET
For applications that require a true high-side switch (so that the LED can remain bright regardless of the Q output impedance) and that need the LED on when the Q output is off, an external P-channel MOSFET controlled by the LOGO! Q output provides a clean solution. The circuit uses a P-channel MOSFET in the anode feed of the LED; the LOGO! Q output pulls the gate low to turn on the MOSFET, and the LED anode is at +24 V whenever the LOGO! output is off. With the LED cathode tied to M through a series resistor, the LED lights when the LOGO! is off and dark when the LOGO! is on.
Component choice: a logic-level P-channel MOSFET with low RDson at Vgs = 4.5 V or less, because the LOGO! Q output swings only 0 V to +24 V. The maximum gate-to-source drive for the P-channel is +24 V, which is the off state. Common parts include:
- Infineon IPP2307N03L G — 30 V, 5 A, RDson = 23 mΩ at Vgs = 4.5 V, TO-220
- Onsemi NTR4101P — 20 V, 2.6 A, logic-level P-channel in SOT-23
- Vishay Si2333DDS — 12 V, 6 A, low-threshold P-channel in SO-8
Wiring steps:
- Connect the P-channel source to the +24 V supply (the same L+ rail that feeds the LOGO! power input).
- Connect the P-channel drain to the LED anode (or to a series resistor followed by the LED anode, depending on layout preference).
- Connect the P-channel gate to the LOGO! Q terminal.
- Add a 100 kΩ pull-down resistor from the gate to M. This forces the gate low (MOSFET on) when the Q terminal is in a high-impedance state, such as during a firmware update or after a stop-mode command.
- Connect the LED cathode to M through a series resistor sized for the target LED current (typically 10–20 mA).
Operation analysis:
- LOGO! Q1 ON (24 V at Q1): Gate = 24 V, source = 24 V, Vgs = 0 V, P-MOSFET off, no current to LED, LED off. ✓
- LOGO! Q1 OFF (high-Z at Q1): Gate pulled low by 100 kΩ to M, source = 24 V, Vgs = −24 V, P-MOSFET on, LED sees +24 V at anode (minus RDson drop, ≈ 0.05 V at 20 mA), cathode at M through Rs, LED on. ✓
Workaround D — Invert the LED Logic in the Program
The simplest fix often requires the smallest change to the wiring: reverse the LED wiring so that the LOGO! Q terminal sources current to the LED in the conventional way, and use the LOGO! program to invert the logic driving Q.
Original intent: LED on when LOGO! command = 0 (output off).
Revised wiring: LED anode to Q1, LED cathode through 2 200 Ω to M. LED on when Q1 is at 24 V (output on).
LOGO! program change: insert a NOT block in front of the Q1 output, or rename the tag and adjust the ladder logic accordingly. In LOGO! Soft Comfort, this is one click on the inverter block; in FBD it is a single NOT gate; in ladder it is a normally-closed contact in the rung driving Q1.
For a panel builder who has already wired 30 LEDs in a common-anode array, this is a significant rework. For a designer in the schematic phase it is the right answer: define the LED logic in the program, not in the wiring. The advantage is that the LOGO! on-state RDson (140 mΩ typical) is so much lower than any external pull-down that the LED is brighter and the panel efficiency is higher.
Diagnostic Procedure
Use this procedure to confirm that a misbehaving LED on a LOGO! transistor output is suffering the "floating cathode" symptom described above, rather than a hardware fault. Required tools: a digital multimeter with 10 MΩ input impedance, a 10 kΩ resistor, and access to LOGO! Soft Comfort in online monitoring mode.
- Power the LOGO! with the 24 V supply. Do not connect any external load to the suspect Q terminal.
- Set the Q output to OFF in the LOGO! program (or force the tag to 0 in monitoring mode).
- Measure the Q terminal voltage with the high-impedance DMM. Expected reading: 18 V to 24 V, not 0 V. If the meter reads 0 V, the terminal is shorted to M or the ITS4140N is damaged; replace the module.
- Force the Q output to ON. The terminal should rise to 24 V minus the small RDson drop (≈ 5 mV at no load, ≈ 0.3 V at 300 mA). If the terminal does not rise, the ITS4140N is in thermal cutoff or the L+ supply is missing.
- Connect a 10 kΩ pull-down between Q and M. The OFF-state terminal voltage should drop to below 1 V. The LED wired in the common-anode configuration should now light at near-full brightness.
- If the LED still does not light, check polarity (LED cathode is the flat side of the epoxy, marked by the shorter lead on through-hole parts), the series resistor value, and the LED forward voltage. A typical 5 mm red LED has Vf = 1.8–2.2 V; if the series resistor is much larger than 2 200 Ω the LED current is too low to illuminate.
- If the LED is partially lit when Q is ON, the pull-down is too small. Increase Rp to 47 kΩ or 100 kΩ to reduce the waste current and verify that the LED current at the Q = 24 V state is below the LED holding current (typically 1–5 mA for indicator LEDs).
- Repeat the measurement on each Q terminal. If only some outputs show the floating-cathode behavior, the issue is panel wiring (open return) rather than the LOGO! silicon.
Troubleshooting Matrix
| Symptom | Likely cause | Verification | Resolution |
|---|---|---|---|
| LED off when Q is off, no pull-down | Floating Q terminal (expected) | Measure Q with DMM, expect 18-24 V | Add Rp or invert wiring |
| LED dim when Q is off, no pull-down | LED Vf at conduction knee, leakage current | Reduce supply to 12 V, LED goes off | Add Rp or invert wiring |
| LED bright when Q is on, with pull-down | Rp too small, waste current > LED holding current | Measure Q = 24 V, current through Rp > 5 mA | Increase Rp to 47-100 kΩ |
| LED flickers with no pattern | LOGO! in run/stop transition, Q toggling | Check program state in monitoring mode | Force Q stable, add small cap (10 nF) across LED |
| All Q outputs read 0 V when off | Module damaged, ESD event, or wiring short | Check L+ supply, replace module | Replace LOGO! base or DM |
| One specific Q output behaves differently | Internal ITS4140N failure | Swap with neighboring Q, test | Replace module if confirmed |
| LED on when Q is on (no pull-down) | Wiring reversed, or LED on +24V side of Q | Trace wiring, verify anode is at Q | Correct wiring or invert program |
Specification Reference Table
| Parameter | LOGO! 8 transistor output | LOGO! 8 relay output |
|---|---|---|
| Catalog example | 6ED1052-1MD08-0BA1 |
6ED1052-1HB08-0BA0 |
| Output topology | High-side switch (ITS4140N) | Potential-free changeover contact |
| Nominal voltage | 24 V DC | 24 V DC / 250 V AC |
| Continuous current per output | 0.3 A | 5 A (DC) / 8 A (AC) |
| Sink capability | None (high-Z when off) | Yes (true SPDT contact) |
| Off-state terminal voltage (unloaded) | Floating, ≈ Vbb via leakage | Open circuit |
| Short-circuit protection | Yes, electronic (ITS4140N) | None — external fuse required |
| Cycle life | Unlimited (solid state) | 100 000 cycles at full load |
| Switching time | ≈ 50 µs turn-on, 100 µs turn-off | ≈ 10 ms (mechanical) |
| On-state resistance | 140 mΩ typ | ≈ 30 mΩ contact |
| Leakage in off state | ≤ 5 µA | 0 µA (open contact) |
| Status feedback | Open-drain fault pin | None |
Firmware and Configuration Notes
The ITS4140N behavior is firmware-independent in the sense that the silicon itself cannot sink; the firmware controls the gate of the high-side switch and nothing else. However, three firmware-related behaviors can interact with the off-state terminal voltage and are worth documenting:
- Stop mode behavior. In LOGO! Soft Comfort versions prior to 8.3, the stop-mode behavior was to latch the last output state. From 8.3 onward, the outputs are forced off on stop. If the application depends on the "off" state being a true high-impedance, verify that the firmware revision matches the expected stop behavior; otherwise the outputs may remain latched at the last commanded state and the LED wiring will not behave as designed.
- PWM outputs (LOGO! 8.2 and later). The PWM function block generates high-frequency switching on the Q output (default 100 Hz, adjustable to 1 kHz). The off-state terminal voltage during the PWM off-phase is high-impedance, but the PWM on-phase looks identical to a normal Q-on state. Diagnostic equipment that samples the Q terminal at low rate will measure an average voltage, not a true 0 V.
- Network failure (LOGO! 8.3 with Ethernet). When the LOGO! loses its network connection, the outputs behave as programmed in the on-disconnect behavior block (default: outputs off). The off state is again a true high-impedance, so the off-state leakage analysis above applies.
The latest LOGO! Soft Comfort version is 8.4 (as of this writing). The most recent system manual revision is referenced at the Siemens LOGO! 8 system manual entry on the Siemens Industry Online Support portal and should be consulted for the specific output-stage schematic of the module in use.
Frequently Asked Questions
Can a Siemens LOGO! transistor output sink current?
No. The Q1–Q4 outputs on LOGO! 8 modules with transistor outputs (catalog variants identifiable by the absence of relay contact symbols and the "T" suffix in the marketing name) are implemented with the Infineon ITS4140N high-side switch. The output can only connect the load to +24 V when commanded on. When commanded off, the terminal is a high-impedance node, not a 0 V rail. Use a relay output module, an external pull-down resistor, an external P-channel MOSFET, or invert the LED logic in the LOGO! program.
Why is my common-anode LED dim or flickering when the LOGO! output is off?
The Q terminal is not at 0 V when commanded off; it is high-impedance and pulled weakly toward +24 V by the ITS4140N body diode and module-internal bias network. The LED sees only 1–3 V of forward drop, not enough to fully light it, and the small leakage current is sensitive to nearby wiring capacitance and to the LED own Vf tolerance. Add a pull-down resistor (2.2 kΩ for 10 mA red LEDs) or invert the LED wiring so that the LOGO! sources current to the LED in the conventional way.
What is the difference between the ITS4140N and a "real" transistor output?
A discrete transistor output (open-collector NPN, open-drain N-channel, or push-pull CMOS) has a defined voltage at the output pin for both states. The ITS4140N is a single-direction high-side switch with built-in protection, charge pump, current limiting, and thermal shutdown. It is optimized for switching 24 V loads to ground, not for general-purpose logic-level signaling. Treat it as a smart relay contact to Vbb, not as a digital logic output.
Can I use the LOGO! Q output to drive an optocoupler input on another device?
Yes, in the source configuration. The optocoupler LED anode goes to the Q terminal, the LED cathode goes through a 4.7 kΩ resistor to M. When the LOGO! output is on, the optocoupler LED conducts at about 5 mA; when the LOGO! output is off, the optocoupler input is high-impedance and the optocoupler is in its off state. Sinking configuration is not supported and would require either a relay output module or an external PNP transistor (P-MOSFET) workaround.
How do I check whether my LOGO! has transistor or relay outputs?
Read the catalog number on the side label. Transistor output modules carry catalog variants with the "MD" designator in the suffix (e.g. 6ED1052-1MD08-0BA1). Relay output modules carry catalog variants with the "HB" or "FB" designator (e.g. 6ED1052-1HB08-0BA0). The "T" in the marketing name (e.g. "LOGO! 24CEoT") also indicates transistor outputs; "R" indicates relay outputs. The module wiring diagram in the LOGO! 8 system manual shows the schematic for the specific variant.