Key Takeaways
- Discrete output families remain relay, transistor, SSR, and triac, with analog modules handling 0-10 V / 4-20 mA.
- IEC 61131-2 (2026 draft) tightens digital output tests and adds an IO-Link conformance profile.
- Modern transistor outputs increasingly use SiC and GaN switches for higher frequency and lower losses.
- Networked output modules (PROFINET, EtherNet/IP, IO-Link) are now standard on mid-range PLCs.
- Safety outputs reach SIL3 / PLe Cat 4 with dedicated safe channels on GuardLogix 5380 and S7-1500F.
Quick read for technicians and buyers selecting the right discrete or analog output module for a 2026 PLC panel.
As PLC professionals selecting discrete or analog output modules in 2026, you face more options than the four legacy categories suggest. Relay, transistor, SSR, and triac outputs are still the workhorses of every control panel, but the rules around them have shifted with the 2026 revision of IEC 61131-2, the rise of IO-Link, and the introduction of SiC and GaN semiconductor switches. This note updates our 2024 reference for technicians, panel builders, and sourcing teams, and shows where to cross-reference and request a quote via KOEED.
What changed since 2024
Three shifts deserve attention before any selection work in 2026:
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IEC 61131-2 alignment. The 4th edition draft circulating in 2026 tightens the 24 V DC test levels for digital outputs, adds an explicit IO-Link conformance profile, and reclassifies test categories for solid-state and relay outputs.
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Wide-bandgap semiconductors. SiC and GaN transistors are entering mid-range PLC output stages, raising switching frequency into the 100 kHz range and cutting conduction losses compared with traditional silicon MOSFETs.
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Safety by default. Platforms such as Siemens S7-1500F and Allen-Bradley GuardLogix 5380 now ship SIL3 / PLe Cat 4 safe outputs as configurable channels rather than as separate safety relays.
For a deeper look at safety output wiring on a modern platform, see our engineering notes archive or open a free consultation on the KOEED AI Diagnostic Tool.
Relay output units
Relay outputs remain the universal choice where voltage and current ranges span DC and AC, and where galvanic isolation is mandatory. In 2026 you will still see them on entry-level CPUs like Mitsubishi FX5U and on legacy retrofits for PLC-5 and SLC 500. Mechanical life is the trade-off: spec sheets today publish 100,000 to 1,000,000 cycles at rated load, so plan replacement if your actuator switches more than once per minute.
! Warning
Never share a relay output contact between an inductive DC load and a low-level signal input. The flyback spike from the inductive load will reset the input module even with a snubber.
Transistor output units
Transistor outputs (NPN sinking or PNP sourcing) are the default on every modern mid-range PLC. In 2026, silicon MOSFETs remain the standard, but SiC and GaN switches are entering high-speed pulse trains on motion modules. Typical sourcing outputs deliver 0.5 A per channel at 24 V DC, with short-circuit and overload protection built into the module. For Mitsubishi MELSEC iQ-R, Omron NJ/NX, and Schneider Modicon M580, sourcing is the convention; check the polarity before wiring.
Solid-state relay (SSR) output units
SSR outputs combine the optical isolation of a transistor output with a triac or back-to-back SCR stage, so they switch AC loads silently and reliably. By 2026 most manufacturers publish zero-cross switching for resistive loads and random-turn-on for inductive loads. SSR is the right choice for temperature controllers, solenoid valves, and heater banks where mechanical wear would otherwise dominate the maintenance schedule.
Triac output units
Triac outputs are bidirectional AC switches, ideal for lamp dimmers, contactor coils, and small motor starters. The 2026 revision of IEC 61131-2 introduces tighter EMI tests for triac outputs to limit conducted noise on 230 V AC lines. If you are spec'ing a triac output next to an analog input, plan for a snubber and physical separation in the cabinet.
Analog output units
Analog outputs in 2026 still center on 0-10 V and 4-20 mA, but 16-bit resolution is now mainstream on mid-range modules and 18-bit is appearing on premium motion controllers. Both voltage and current outputs should be protected against short circuits and over-voltage on the field side. Use the PLC Analog Calculator when scaling engineering units to raw counts.
Networked and safety outputs (2026 standard)
Two output families have become first-class citizens in 2026:
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IO-Link outputs. Single-channel digital outputs with bidirectional diagnostics, configured over IO-Link masters on platforms from Siemens ET200SP and KEYENCE vision systems.
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Safety outputs. SIL3 / PLe Cat 4 channels on GuardLogix 5380, S7-1500F, and Omron NX-S safety modules.
Output circuits at a glance
| Circuit |
Switching element |
Typical load |
IEC 61131-2 (2026) notes |
| Relay |
Electromechanical contact |
Universal DC / AC, 2 A |
Life-cycle test tightened |
| Transistor |
MOSFET / SiC / GaN |
DC 24 V, 0.5 A |
New short-circuit class |
| SSR |
Opto-triac / SCR |
AC 24-230 V, 1-2 A |
dv/dt limit clarified |
| Triac |
Bidirectional triac |
AC lamp / coil |
Conducted EMI test |
| Analog |
16-bit DAC |
0-10 V / 4-20 mA |
Resolution class added |
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Frequently asked questions
What is the difference between relay, transistor, and SSR outputs?
Relay outputs use a mechanical contact to switch AC or DC; transistor outputs use a semiconductor to switch only DC; SSR outputs use an opto-isolated triac to switch AC silently.
Are SSR outputs suitable for inductive AC loads?
Choose a random-turn-on (phase-control) SSR over a zero-cross device. Zero-cross SSRs let inrush current peak exceed rated values; random-turn-on SSRs delay turn-on until voltage crosses zero.
Do 2026 PLCs still ship triac output modules?
Yes, on entry-level CPUs such as Siemens LOGO! and Schneider Zelio Logic. Triac outputs are still the lowest cost option for AC lamp and contactor control.
Which output family should I choose for a 24 V DC solenoid?
A PNP sourcing transistor output is the modern default. It is faster than a relay, immune to contact wear, and integrates with short-circuit diagnostics on most 2026 mid-range PLCs.
Can I source obsolete output modules like 1746-OB16 or 6ES7 322?
Yes. KOEED keeps active and EOL stock side by side for Allen-Bradley, Siemens, and other legacy families. Send the part number to Moritta@KOEED.COM for a quote.
