Interfacing Playbook: Modern ↔ Legacy#

This is the capstone for the Retro & Legacy section. Everything else in this section — era constraints, electrical assumptions, legacy signaling, aging behavior — leads here: actually connecting a modern microcontroller, SBC, or USB device to an older system without destroying either one.

This page is the systematic playbook: how to characterize an unknown interface, identify what can damage modern hardware, select the right protection and conversion topology, and bring it up safely on the bench.

Step 1: Domain Identification Checklist#

Before designing any interface circuit, characterize both sides of the connection. Skipping this step is how MCU pins get destroyed.

Voltage and Current#

• What is the signal voltage range? Measure or look up the output levels – not just the nominal, but the worst case. RS-232 can swing +/-15V. A “24V” industrial output may be 28V with the relay off and 0V with the relay on. A “5V logic” signal from a 1970s system may actually sit at 4.2V due to TTL output characteristics
• What is the supply voltage range? Including startup, shutdown, and fault conditions. A “12V” automotive system is 6-80V in practice
• What current does the signal source deliver? A 4-20 mA loop is a current source. An RS-232 driver can source/sink 10-20 mA. A relay contact closure delivers whatever the pull-up circuit provides. The interface must not load the source beyond its capability
• What current does the load expect? A relay coil draws 50-150 mA. A legacy input may expect milliamps of drive current (TTL inputs sink ~1.6 mA in the low state). A CMOS input draws essentially zero DC current

Impedance#

• Source impedance? A 4-20 mA transmitter is a high-impedance current source. An RS-232 driver is a low-impedance voltage source (typically 300-600 ohms output impedance). A thermocouple is a very low voltage source with ~10 ohm source impedance
• Load impedance? The input impedance of the receiving circuit determines how much it loads the source. A CMOS microcontroller input is >10 Mohm at DC. An ADC input may have a sample-and-hold capacitor that draws transient current. A legacy audio input may be 10 kohm or 600 ohm depending on era and design

Grounding Model#

• Is the signal ground-referenced, floating, or differential? A ground-referenced signal has one terminal at system ground and the signal on the other. A floating signal (like a thermocouple or a current loop) has no inherent ground reference. A differential signal (RS-485, balanced audio) is defined as the voltage between two wires, independent of ground
• Where is ground? Legacy equipment may reference to chassis (connected to mains earth), to a signal ground bus (which may or may not be connected to earth), or to nothing identifiable (floating). Measure the voltage between the signal ground of the legacy system and the ground of the modern system before connecting them. If there is more than a volt or two, isolation is required
• Is the chassis grounded through the mains plug? Equipment with a 3-prong plug has its chassis at earth potential. Equipment with a 2-prong plug may float. Battery-powered equipment floats. This determines whether connecting signal grounds creates a ground loop through the mains wiring

Isolation Requirements#

• Safety isolation? If the legacy system operates at mains voltage or connects to mains-referenced circuits, safety-rated isolation (reinforced insulation, creepage/clearance, certified components) is required. This is non-negotiable
• Noise isolation? If connecting the grounds of both systems creates unacceptable noise (hum, buzzing, data errors), galvanic isolation breaks the ground loop. This is the most common reason for isolation in audio and instrumentation interfaces
• Fault protection? If miswiring, a component failure, or a surge could put dangerous voltage on the interface, isolation protects the modern equipment from destruction and the operator from shock

Fault Scenarios#

• What happens if the cable is connected backwards? RS-232 TX and RX swapped is annoying but harmless (the drivers can handle it). Power and ground swapped on a sensor cable will destroy the sensor and possibly the MCU
• What happens if the cable is connected while both systems are powered? Hot-plugging creates transient currents through connector capacitance and can inject spikes through ESD and charge redistribution. If the connector does not have ground-first/signal-last pin sequencing, every hot-plug is an ESD event
• What happens if one system is powered and the other is not? If the MCU is off and the legacy system drives a signal into an MCU pin, the signal may backfeed through the MCU’s ESD clamp diodes into the unpowered VCC rail, potentially powering part of the MCU through an unintended path (parasitic power-up). This causes latch-up and damage
• What happens during a fault on the legacy side? A shorted winding in a transformer, a stuck relay, a failed voltage regulator – these can put unexpected voltages on the interface. The interface circuit must survive the worst-case fault voltage without passing damage through to the MCU

Step 2: What Can Kill an MCU?#

Modern CMOS microcontrollers are fragile compared to the legacy systems they connect to. The following conditions destroy I/O pins or create latent damage.

Overvoltage on Any Pin#

CMOS inputs have ESD clamp diodes to VDD and VSS. If a signal exceeds VDD + 0.3V (or goes below VSS - 0.3V), the clamp diode forward-biases and injects current into the supply rail. If the current exceeds the clamp diode’s rating (typically 1-10 mA), the diode fails or heats the die locally.

How it happens at the interface: An RS-232 signal at +12V drives directly into a 3.3V MCU pin. A 24V industrial signal connected through a voltage divider where the divider was omitted and the signal connected directly. A legacy system with a pull-up to 5V connected to a 3.3V MCU that is not 5V-tolerant.

Negative Voltage on Any Pin#

Worse than overvoltage. A negative voltage forward-biases the substrate diode from the pin to VSS, injecting current into the substrate. This can trigger latch-up – a parasitic SCR that creates a low-impedance path from VDD to VSS, drawing excessive current that destroys the chip unless power is removed in milliseconds.

How it happens at the interface: RS-232 negative swings (-3V to -15V). Inductive kick from a relay coil or motor. A legacy signal that swings below ground as part of its normal operation (bipolar signals, AC-coupled signals). Ground offset between systems – if the legacy system’s ground is 2V below the MCU’s ground, a 0V signal from the legacy system appears as -2V to the MCU.

Ground Offset#

If the two systems do not share a common ground, or if the shared ground has significant impedance, the “ground” potential of the legacy system may differ from the MCU’s ground by hundreds of millivolts to volts. Every signal from the legacy system appears shifted by this offset when measured by the MCU.

What it damages: If the offset is large enough that legacy signals go below the MCU’s VSS, negative voltage appears on the input pins (see above). Even if it does not cause direct damage, the offset shifts all signal levels and can push nominally-valid signals outside the MCU’s input thresholds.

Inductive Kick#

Any inductive load (relay coil, solenoid, motor) produces a voltage spike when current is interrupted. If the MCU is driving the load (through a transistor or MOSFET) and the flyback protection fails or is absent, the spike appears at the drain/collector and can couple into the MCU through parasitic capacitance, through the ground plane, or directly if the layout is poor.

How it happens at the interface: MCU drives a relay through an N-channel MOSFET. The flyback diode is absent, wrong polarity, or placed too far from the coil. The relay’s inductive kick generates a 200V spike that exceeds the MOSFET’s drain rating, avalanches the MOSFET, and couples into the MCU.

Backfeeding Through I/O Pins#

When the MCU is unpowered but an external signal is applied to an I/O pin, current flows through the pin’s ESD clamp diode into the VDD rail. This partially powers the MCU through an unintended path — some internal circuits power up, others do not, and the resulting state can cause latch-up or permanent damage.

How it happens at the interface: The legacy system is powered on before the MCU. A cable is connected while the MCU is off. The MCU’s supply is disabled by a regulator enable signal, but signals from the legacy system are still present on the I/O pins.

Prevention: Add series resistors (1-10 kohm) on I/O pins to limit backfeed current below the clamp diode’s rating. Alternatively, use interface circuits with their own supply enable that tri-state outputs when the MCU is unpowered.

Step 3: Common Adapter Building Blocks#

These are the standard circuit patterns for bridging domains. Mix and match based on the domain identification checklist results.

Level Shifting#

Unidirectional, high-to-low (most common case):

• Resistive voltage divider — two resistors, works for signals up to ~1 MHz. Output impedance is the parallel combination of the two resistors, which limits drive strength and bandwidth
• Series resistor + clamp to VDD – a resistor (1-10 kohm) in series with the signal, relying on the MCU’s internal clamp diode to limit voltage to VDD + 0.3V. The resistor limits clamp current to safe levels. Simple and effective for slow signals where the clamp current is milliamps or less

Unidirectional, low-to-high:

• Open-drain/open-collector output with pull-up to the higher voltage. The MCU pulls the line low; the pull-up brings it to the legacy system’s voltage when the MCU releases it. Works for slow signals and buses
• Buffer IC powered from the higher voltage with input compatible to the lower voltage (e.g., 74HCT series accepts 3.3V inputs when powered from 5V, because TTL input thresholds are low enough)

Bidirectional:

• BSS138 MOSFET translator circuit — the standard for I2C and other open-drain bidirectional buses. One MOSFET per signal line, pull-ups on both sides. Documented in NXP AN10441
• Dedicated bidirectional translator ICs (TXB0108, TXS0102) — faster and smaller than the MOSFET circuit, but problematic with open-drain buses and slow edges

Buffering and Impedance Bridging#

When the source impedance and load impedance are mismatched, or when the source cannot drive the load without distortion:

• Unity-gain buffer (op-amp voltage follower): High input impedance, low output impedance. Useful for bridging a high-impedance legacy source (like a tube-era audio output) to a low-impedance modern input. Choose the op-amp for the voltage range (rail-to-rail if the signal approaches the supply rails) and bandwidth
• Emitter follower / source follower: The discrete equivalent. A single transistor provides current gain without voltage gain. Simple, fast, and works at voltages above the MCU’s supply
• Series termination resistor: For transmission-line effects on long cables (>1/10 wavelength at the signal’s edge frequency). A resistor at the source matching the cable impedance prevents reflections. Standard on RS-485, Ethernet, and any interface designed for cable runs

Galvanic Isolation#

Use when the domain identification checklist reveals safety risk, ground loops, or ground offsets that exceed the MCU’s tolerance range.

Optocouplers: Cheapest isolation for slow signals (<50 kHz for standard parts, up to 25 Mbaud for high-speed types). LED drive current degrades over time, so design with CTR margin. Best for: relay replacement, slow digital signals, isolated status indicators.

Digital isolators (capacitive or magnetic): Faster (up to 150 Mbaud), lower power, no degradation over time. Available with integrated isolated power. Best for: SPI, I2C, UART, CAN, any high-speed digital interface where isolation is needed.

Isolation transformers: Best for AC signals (audio) and power transfer. Provide inherent common-mode rejection for the signal. Audio isolation transformers break ground loops without digitizing the signal. Best for: audio interfaces, Ethernet (already transformer-isolated by design), pulse/gate drive.

Isolated DC-DC converters: When the isolated side needs its own power rail, a small isolated DC-DC module (Murata NME, Recom R1S) generates a secondary supply with galvanic isolation. Common in RS-485 interfaces, 4-20 mA loop adapters, and anywhere an interface circuit needs an isolated power domain. Without a truly separate supply, isolation is only on paper – if both sides of an optocoupler share the same non-isolated DC-DC, the ground paths reconnect through the shared supply.

When to use which:

Protection Components#

At the interface boundary – between the connector and the translation/isolation circuit:

• Series resistors (100 ohm - 10 kohm): Limit current during fault conditions. A 1 kohm resistor between a legacy output and an MCU input limits current to 12 mA at 12V – within the ESD clamp’s tolerance for most parts. Also forms an RC filter with the pin capacitance, slowing edges and reducing EMI. This is the single most effective protection component and should be on every interface pin
• TVS diodes: Clamp fast transients (ESD, inductive kick) that the series resistor cannot absorb alone. Bidirectional TVS for signals that swing both ways; unidirectional for DC-only. Place between the signal line and ground, after the series resistor, as close to the connector as possible
• Schottky clamp diodes to supply rails: External versions of the MCU’s internal ESD clamps, but rated for higher current. Schottky diodes (BAT54S in a dual common-cathode package) from the signal to VDD and VSS clamp overvoltage and undervoltage
• Fuses or PTC resettable fuses: On power lines at the interface boundary. Protect the MCU’s supply from a short or overvoltage on the legacy side. A PTC resets automatically after the fault clears
• RC snubbers: Across relay contacts or any mechanical switch interface. A series RC (100 ohm + 100 nF typical) damps contact arcing and reduces the EMI transient that the MCU sees

Protocol Conversion#

Physical-layer and framing differences between legacy and modern:

Step 4: Grounding and Shielding Patterns#

Ground strategy is the most commonly botched aspect of legacy interfacing. Getting this wrong makes everything else noise.

Single-Point Grounding#

Connect the grounds of the legacy and modern systems at exactly one point. All signal returns flow through that single connection. This prevents ground loops by ensuring there is only one path for ground current.

When it works: Low frequencies (audio, slow digital), short cable runs, systems where all ground connections are under control.

When it fails: High frequencies (above ~1 MHz), where the inductance of the single-point ground wire becomes significant and forces return currents to find other paths (through capacitive coupling, through cable shields, through the mains ground).

Shield Termination#

Cable shields must be connected to something – the question is where and how:

• Audio and low-frequency analog: Connect the shield at the source end only. This drains static charge and provides electric-field shielding without creating a ground loop through the shield. The receiving end’s shield floats (or is connected through a small capacitor to ground for RF shielding without DC ground loop)
• RF and high-frequency digital: Connect the shield at both ends. At high frequency, the shield must be a low-impedance return path for the signal’s return current. Connecting at one end only creates an antenna above the shield’s quarter-wavelength frequency
• Mixed (common in legacy systems): Connect at the source end for audio, both ends for data. If a cable carries both audio and data signals, the compromise is to bond the shield at both ends and break the ground loop elsewhere (isolation transformer on the audio, differential signaling on the data)

Chassis Bonding#

When the legacy system uses chassis ground as signal reference (common in industrial equipment, older audio gear, and test instruments):

• Bond the modern system’s ground to the legacy chassis ground at the interface point
• Keep the bond short and low-impedance (wide braid or strap, not a long wire)
• Do not create additional ground connections elsewhere – one point of contact between the two ground systems
• If the chassis bond creates unacceptable noise, insert isolation at the signal level instead of attempting to lift the ground

Step 5: Worked Mini-Patterns#

These are conceptual interface patterns – not complete schematics, but the essential topology for each common scenario.

MCU ↔ RS-232#

The problem: RS-232 swings ±3V to ±15V. MCU UART is 0/3.3V. Direct connection destroys the MCU.

Pattern:

RS-232 device                              MCU (3.3V)
  TX ────── MAX3232 R_IN ─── R_OUT ──── UART RX
  RX ────── MAX3232 T_OUT ── T_IN ───── UART TX
  GND ───── MAX3232 GND ─────────────── GND
                     │
                   3.3V (C1–C4 charge pump caps per datasheet)

• The MAX3232 handles all level translation and generates ±6V from 3.3V using internal charge pumps
• For one-off bench connections, a USB-serial adapter (FTDI, CP2102) replaces all of this with a single module
• If the RS-232 device is far away (long cable), add TVS diodes (SM712 or similar) on the RS-232 side for transient protection

Watch for: Null modem vs. straight-through wiring. If the legacy device is DTE (like a PC), it expects DCE pinout on the cable – TX on pin 3, RX on pin 2. If both devices are DTE, a null modem crossover (TX to RX, RX to TX) is needed. Getting this wrong does not cause damage but nothing communicates.

MCU ↔ 4–20 mA Current Loop#

The problem: The legacy transmitter outputs a current proportional to a measured value (4 mA = 0%, 20 mA = 100%). The MCU needs to read this as a voltage with its ADC.

Pattern (receiver, loop-powered by external supply):

Loop supply (+24V) ─── 4–20 mA transmitter ──┬── MCU ADC input
                                               │
                                          250Ω (precision)
                                               │
                                              GND (shared)

• The 250 ohm sense resistor converts 4-20 mA to 1-5V, which is within the ADC’s input range on a 3.3V MCU with a 5V-tolerant ADC (or use a lower value resistor for lower voltage range)
• Use a precision resistor (0.1% tolerance, low tempco) – the resistor’s accuracy directly affects measurement accuracy
• Add a TVS diode across the sense resistor to clamp voltage if the loop faults to a higher current
• Add an RC filter (1 kohm + 100 nF) before the ADC input if the loop picks up noise from long cable runs

For isolated measurement: Insert an analog isolation amplifier (AMC1200, ISO124) between the sense resistor and the ADC, with an isolated DC-DC module powering the loop side. This breaks the ground loop between the MCU and the loop supply.

Watch for: A broken wire in a 4-20 mA loop reads 0 mA. The 4 mA “zero” is specifically chosen so that a broken wire (0 mA) is distinguishable from a minimum reading (4 mA). Firmware that treats 0 mA as a valid reading instead of a fault will miss broken sensors.

MCU ↔ Relay Outputs / Contact Closures#

The problem: Legacy industrial equipment uses relay contacts to indicate status (dry contacts) or drives relay coils at 12–24V. The MCU needs to read contact closures as digital inputs and drive relay coils as digital outputs.

Reading a contact closure:

MCU 3.3V ─── 10kΩ pull-up ──┬── MCU GPIO (input, no internal pull-up)
                              │
                         Contact ── GND (legacy side)

• The pull-up holds the GPIO high when the contact is open. The contact pulls it low when closed
• If the contact is on a separate ground from the MCU, use an optocoupler instead of a direct connection – the legacy side drives the opto LED, the MCU reads the phototransistor
• Add hardware debouncing (100 nF cap from GPIO to ground) or debounce in firmware (10-50 ms timer after first edge). Mechanical contacts bounce for 1-20 ms

Driving a relay coil:

MCU GPIO ── 1kΩ ── NPN base ──┐      Relay coil
                               │         │
                            NPN (2N2222)  │
                               │         │
                              GND    +12V/+24V
                                         │
                              Flyback diode (1N4148)
                              across coil (cathode to +V)

• The NPN transistor (or N-channel MOSFET) switches the relay coil. The MCU GPIO drives the base/gate through a series resistor
• The flyback diode is non-negotiable. Without it, the inductive kick when the relay opens will exceed the transistor’s voltage rating and potentially couple back into the MCU
• For MOSFET drive: a logic-level MOSFET (IRLML2502 or similar) does not need the base resistor. Gate direct from GPIO (or through a small series resistor for EMI), source to ground, drain to relay coil

MCU ↔ Unbalanced Audio Input/Output#

The problem: Legacy audio equipment uses unbalanced (single-ended) connections at line level (~1V RMS, ±1.4V peak) referenced to chassis ground. The MCU’s ADC is DC-coupled, unipolar (0 to VDD), and sensitive to overvoltage.

Audio input to MCU ADC:

Audio source ── AC coupling cap (1µF) ──┬── voltage divider ── MCU ADC
                                         │
                                    bias resistors
                                    (set DC midpoint
                                     at VDD/2)

• The coupling capacitor blocks DC offset from the audio source
• A resistive divider from VDD to GND sets the DC bias point at VDD/2 (e.g., two 10 kohm resistors from 3.3V to GND with the midpoint connected to the signal through a 10 kohm resistor)
• The audio signal swings symmetrically around VDD/2. At ±1.4V peak into a 2:1 divider (if needed), the signal stays within 0–3.3V
• Add a Schottky clamp to VDD and GND if overvoltage is possible (guitar amp output, legacy mixer with high output levels)

MCU DAC/PWM to audio output:

MCU DAC ── series resistor (1kΩ) ── AC coupling cap (10µF) ── output jack
                                                                    │
                                                                   GND

• The series resistor sets the output impedance. Legacy equipment expects either 600 ohm or high-impedance (>10 kohm) output
• The coupling capacitor blocks DC. For audio-frequency response down to 20 Hz into a 10 kohm load, the cap needs to be at least 1/(2pi x 20 x 10000) = 0.8 uF – so 1-10 uF is typical
• If using PWM, add an LC low-pass filter after the series resistor to remove the PWM carrier. Cutoff frequency above 20 kHz, well below the PWM frequency

Watch for: Audio grounds between equipment. Connecting the MCU ground to the audio equipment ground may create a ground loop with audible hum. If so, insert an audio isolation transformer (Triad Magnetics TY-250P or similar 600:600 ohm) in the signal path.

Tips#

• Place a series resistor (100 ohm to 1 kohm) on every interface pin as a default practice – this single component limits fault current, forms an RC low-pass with pin capacitance, and reduces EMI coupling at negligible performance cost for signals under 1 MHz
• Complete the domain identification checklist before building any interface circuit – characterizing both sides of the connection up front prevents the most common MCU-destroying mistakes
• Use isolated DC-DC modules (Murata NME, Recom R1S) to power the isolated side of optocoupler or digital isolator circuits – without a truly separate supply, isolation is only on paper
• Test the complete interface with realistic cable lengths and with both systems powered from their intended sources, not just bench supplies – field conditions differ significantly from bench conditions

Caveats#

• “Works on the bench” does not mean it works in the field – Bench conditions are clean: short cables, common ground through the bench supply, no nearby motors or relays. In the field, cables are long, grounds differ, and EMI is real. Test with realistic cable lengths and with both systems powered from their intended sources
• Isolation does not help if the isolated supply shares the same input power – If both sides of an optocoupler are powered from the same wall adapter through a non-isolated DC-DC module, there is no real isolation – the ground paths connect through the shared supply. True isolation requires separate power domains or a certified isolated DC-DC converter
• Legacy systems assume their loads are passive – A legacy 4-20 mA transmitter expects to drive a passive 250 ohm load, not an active circuit that might inject current back into the loop. A tube-era audio output expects a resistive load, not a capacitive one that reflects energy. Respect the legacy system’s assumptions about what it is driving
• Hot-plugging legacy connectors is never safe unless designed for it – DB-9, DB-25, DIN, screw terminals, banana plugs – none of these guarantee ground-first contact. Every hot-plug event is a potential ESD event, ground bounce event, and signal transient. Power down before connecting
• Auto-direction level translators (TXB0108 family) fail on buses with pull-ups – They interpret the pull-up as a driven signal and fight it, causing oscillation or stuck states. Use MOSFET-based translators for I2C, 1-Wire, or any open-drain bus
• Optocoupler CTR degrades over time – A circuit that works with a new optocoupler may fail in 3-5 years as the internal LED dims. Design the drive current with 2-3x margin over the minimum required, or use digital isolators, which have no degradation mechanism
• Ground voltage between systems can drift with load – A DMM may show 0V between the grounds at idle, but once the interface is active, 500 mV of offset appears because the interface cable carries return current through a high-impedance path. Measure the ground offset with the interface active and current flowing, not just at idle
• USB ground is not isolated – USB-connected devices share ground with the host PC. If the legacy system has a ground offset relative to the PC, that offset appears across the interface. A USB isolator module (such as one based on the ADUM4160) breaks this connection

In Practice#

• An MCU that resets or locks up when a relay on the same board switches likely has insufficient flyback protection or poor ground routing – check the flyback diode polarity and placement, and monitor the supply rail on a scope during relay switching
• A 4-20 mA reading that shows 0 mA indicates a broken wire or dead transmitter, not a zero measurement – firmware should flag any reading below 4 mA as a fault condition
• Audible hum (50/60 Hz) appearing when the MCU ground connects to legacy audio equipment indicates a ground loop – an isolation transformer in the audio path or a USB isolator on the digital side breaks the loop
• A level-translated I2C bus that shows erratic behavior or stuck lines often indicates an auto-direction translator (TXB0108 family) fighting the bus pull-ups – replacing with a BSS138 MOSFET translator circuit resolves the issue

Bench Bring-Up Checklist#

Follow this sequence when connecting a modern MCU to a legacy system for the first time.

Before Connecting Anything#

• Complete the domain identification checklist above – know the voltage range, current levels, impedance, and grounding model of both sides
• Measure the voltage between the two systems’ grounds (with both powered, if applicable). If it exceeds 1V, plan for isolation
• Verify the interface circuit has protection on every pin that connects to the legacy system (series resistors at minimum, TVS/clamps if the fault analysis warrants it)
• Verify flyback diodes are installed on every inductive load the MCU drives
• Verify the power supply for the interface can handle the current required by isolation components, relay coils, LED drive, etc.

First Connection — Unpowered#

• Connect the cable with both systems powered off
• Visually verify the wiring – especially that power and ground are on the correct pins. RS-232 pin numbering is a common source of errors (DB-9 pin 5 is ground, but DTE and DCE have different pin assignments for TX/RX)
• Measure continuity on each signal path from the legacy connector through the interface circuit to the MCU pin. Confirm no shorts between adjacent pins

Power-Up Sequence#

• Power up the MCU side first. Verify the interface circuit is alive and all protection voltages are correct (3.3V rail, any isolated supply rails)
• Monitor the MCU’s power supply current. If it spikes when the legacy system is connected (even unpowered), current is backfeeding through the interface
• Power up the legacy system. Monitor the MCU’s I/O pins on the scope during legacy power-up – look for voltage excursions outside the MCU’s supply rails
• Verify signal levels at the MCU’s I/O pins match expectations (correct voltage range, correct logic polarity, no unexpected noise)

Functional Test#

• Start with read-only operations. When reading a sensor, verify the value makes sense. When reading a contact closure, toggle the contact and watch the GPIO
• Add write operations one at a time. Drive one relay, send one RS-232 character, output one DAC sample. Verify the legacy system responds correctly
• Monitor power supply current and temperature throughout. A gradually increasing current or a warm component indicates a problem developing
• Test fault scenarios: disconnect the cable while running (if hot-plug is a possibility in the field), toggle power on the legacy side, and verify the MCU recovers without damage or lockup