PLC MODULE 6ES7222-1BF22-0XA8 6ES7 222-1BF22-0XA8 6ES7222-1BF22-OXA8

Siemens 6ES7222-1BF22-0XA8 | SIMATIC S7-200 CN EM 222 — 8 Transistor Outputs 24VDC, 0.75A, Short-Circuit Protected, S7-22X CPU Only, CE Approval, China Production

Overview

The Siemens 6ES7222-1BF22-0XA8 is the eight-channel, 24V DC transistor output expansion module for the SIMATIC S7-200 CN micro-PLC platform.

It is the solid-state counterpart to the relay output EM 222 — where the relay version (6ES7222-1HF22-0XA8) switches loads through mechanical contacts at millisecond speeds with voltage-neutral isolation, the transistor version switches 24VDC loads through semiconductor transistors at microsecond speeds with built-in electronic fault protection.

The transistor versus relay decision is one of the first output selection choices in any PLC expansion design, and understanding it correctly is more important than any other specification difference between the two modules.

A transistor output switches in the range of 100–300 microseconds — roughly 10,000 times faster than a relay contact.

For outputs that must generate pulses (step motor control, flow meter simulation, PWM speed control), this speed is not optional — it is a physical requirement that only solid-state outputs can meet. 

A relay contact simply cannot open and close fast enough to produce a clean pulse at frequencies above a few tens of Hz.

The electronic short-circuit protection in the transistor EM 222 adds a fault tolerance layer that relay outputs cannot match.

When a wiring fault creates a short circuit on a transistor output, the semiconductor's current-sensing circuit detects the excess current and shuts off the affected channel before thermal damage occurs. The transistor recovers once the fault is cleared; the module continues operating on all unaffected channels throughout the fault event. 

In contrast, a short circuit on a relay output relies entirely on the external fuse to clear the fault — the relay contact may be damaged in the process, requiring module replacement.

Key Specifications

Where 0.75A per Output Takes You — Load Matching in Practice

The 0.75A output current rating defines what loads this module can drive directly. Working through common 24VDC load types:

Standard 24VDC relay coils draw 50–100mA — well within the 0.75A limit, and each transistor output can drive one relay coil with significant headroom. If multiple coils must be energised simultaneously on related outputs, the total current stays comfortably within the module's supply limits.

Typical 24VDC solenoid valves draw 200–400mA on actuation.

The 0.75A transistor output handles these with margin to spare, and the high switching speed is often an advantage — pneumatic valves on assembly and packaging machines may cycle dozens of times per minute, far exceeding relay contact life at this duty rate.

24VDC indicator lamps draw 10–50mA; LED pilot lights draw under 30mA.

Transistor outputs drive these effortlessly, and for modulated indicator signals (flashing at defined rates, intensity-controlled LED dimming via PWM), the transistor's fast switching is essential to generate clean waveforms.

Loads drawing more than 0.75A — small DC motors operated directly, high-power solenoids, large contactors with high-current 24VDC coils — exceed the transistor output's rating and must be interfaced through an interposing 24VDC relay sized for the load.

Built-In Electronic Short-Circuit Protection — Practical Fault Tolerance

The short-circuit protection in the transistor EM 222 operates as a self-resetting current limiter. When the output current exceeds the trip threshold — caused by a short-circuit in the field wiring or a shorted load — the protection circuit reduces or interrupts the output current.

The channel typically remains inactive until the PLC program cycles the output (turns it off, then back on), which re-enables the transistor if the fault has been cleared.

In a production environment, this behaviour is valuable for two reasons.

First, the module is not damaged by the fault — unlike a relay contact that may be welded or eroded by unprotected short-circuit current, the transistor's protection prevents thermal damage. Second, other channels continue operating normally — a wiring fault on one output does not affect the remaining seven channels or the rest of the machine's I/O.

The faulty channel can be identified and investigated while the machine continues running on the unaffected outputs.

This contrasts with the relay EM 222, where a short circuit depends on an external fuse for protection, and the relay contact may be damaged before the fuse clears.

In applications where output wiring faults are a realistic operational risk — dusty environments, machines with frequently changed tooling or fixtures, or systems where wiring connectors are routinely disconnected — the transistor module's self-protecting outputs represent a meaningful operational advantage.

S7-200 CN Production — China Variant Context

The "-0XA8" suffix identifies this as a China-produced S7-200 CN variant, manufactured in China for markets accepting CE certification.

It is functionally equivalent to the European-produced 6ES7222-1BF22-0XA0 in all electrical specifications, I/O count, switching characteristics, and S7-22X bus compatibility. Both variants use the identical STEP 7 Micro/WIN programming environment and require no differentiation in the PLC program.

The S7-200 CN (China) variants emerged as Siemens adapted the S7-200 platform for production and distribution in markets where CE approval was sufficient and where the cost economics of local production made sense.

The "CN" designation signals the production variant but carries no functional implication for the engineer installing and commissioning the module.

The XA8 and XA0 variants can coexist in the same S7-22X expansion chain without any incompatibility.

FAQ

Q1: Can the 6ES7222-1BF22-0XA8 transistor outputs generate the step pulses needed for stepper motor control?

The transistor outputs can switch at frequencies adequate for many stepper motor applications. However, Siemens specifically designed the S7-200 CPU's onboard PTO (Pulse Train Output) function — available on Q0.0 and Q0.1 of the S7-22X CPUs — for high-performance stepper motor pulse generation, with dedicated hardware support for precise frequency control and acceleration/deceleration profiles.

For primary step pulse generation, the CPU's dedicated PTO outputs should be used.

Expansion module transistor outputs are appropriate for auxiliary stepper signals (direction, enable, reset) or for secondary axes where simpler control is acceptable.

Q2: Are the transistor outputs PNP (sourcing) or NPN (sinking), and does this affect compatibility with field devices?

The S7-200 EM 222 transistor output module produces PNP (sourcing) outputs. When active, each output transistor connects the output terminal to the 24V positive supply — current flows from the output terminal, through the load, to the 24V supply's negative (0V) terminal.

This PNP sourcing configuration is compatible with the large majority of 24VDC industrial loads designed for positive-signal-switching (PNP) digital inputs.

Equipment or sensors requiring NPN (sinking) output signals — where the active state connects the output to 0V rather than to 24V — require an interposing relay or signal converter to interface correctly with the PNP transistor output.

Q3: How does the short-circuit protection reset after a fault — automatically when the fault clears, or does the PLC program need to act?

The exact recovery behaviour depends on the protection circuit design and the severity of the fault. In most implementations, after a short-circuit protection trip, the output remains off even if the fault is cleared.

The PLC program must cycle the output (write "0" then "1" to the output bit) to re-enable the transistor. A brief power cycle of the expansion module's load supply can also reset the protection.

Before resetting, the field wiring and load should be inspected to confirm the short circuit has been corrected — re-enabling a transistor output into a persisting short circuit will immediately re-trip the protection and may eventually cause thermal damage if the protection circuit is exercised repeatedly.

Q4: Can the EM 222 transistor module be mixed with the EM 222 relay module in the same S7-22X expansion chain?

Yes. The S7-22X expansion bus accepts any combination of digital input, digital output (relay or transistor), and analog expansion modules within the CPU's total expansion module limit (7 modules for S7-224/224XP/226, 2 for S7-221/222).

The relay EM 222 and transistor EM 222 can occupy adjacent positions in the same expansion chain, and the PLC program addresses their outputs identically — consecutive output bytes (QB) regardless of whether the underlying output type is relay or transistor.

This mix-and-match capability allows the machine designer to assign relay outputs to AC loads and infrequent-cycle actuators, and transistor outputs to high-speed DC loads, within a single, compact expansion chain.

Q5: The S7-200 CN is discontinued. How long will replacement EM 222 transistor modules be available?

Siemens formally discontinued the S7-200 and S7-200 CN platforms, with the end of active sales occurring progressively over several years.

While Siemens no longer supplies these modules through its standard sales channels, the installed base of S7-200 CN equipment worldwide is substantial, and the secondary market — industrial surplus dealers, refurbishers, and specialist legacy automation suppliers — maintains stocks. 

For sites with significant S7-200 CN installations, the practical approach is to identify critical spare modules (including the EM 222 transistor and relay variants), assess current stock levels, and procure additional units to cover the expected remaining equipment lifetime from available surplus stocks.