A while back I came across a buyer who had just picked up a fanless industrial PC: ten Ethernet ports — eight of them PoE+ — an Intel CPU, and about 23 W of idle draw. His roadmap was a NAS, some Docker experiments, maybe a router. He already ran Home Assistant on a Raspberry Pi at home, so the automation itch was clearly there.
Nothing on his list involved the terminal blocks. You know, the ones labeled DI and DO.
If you're specifying a fanless industrial PC for a CCTV-heavy project, you probably do the same math: CPU for the streams, ports for the cameras, storage for the footage. Then a question lands from the field: "Can it trip the siren when the fence is cut? Release the doors when the fire panel trips? Tell us the pump room is flooding?" That's the moment digital I/O stops being a spec-sheet footnote and becomes the reason you bought a fanless industrial PC in the first place.
Here's how DI and DO actually earn their keep in alarm and automation work — with wiring you can copy.
A digital input (DI) reads one of two states: closed or open. A digital output (DO) switches one of two states. That's it. The value is in what you hang on them.
DI is for anything with a dry contact: door and window contacts, PIR motion sensors, IR fence beams, smoke-detector relays, emergency buttons, water-leak probes, and the "door forced / held open" relay output of an access controller.
DO is for anything you switch: sirens, strobes, magnetic locks and door strikes (through a relay), gates, camera spotlights, exhaust fans, shutdown signals.
Cameras watch. DI senses. DO acts. A security system that can see but not react is just footage for the after-action report.
On a fanless industrial PC, the sense–decide–act loop closes in software, locally:
A DI channel reports the fence beam is broken.
Your logic — a Node-RED flow, a few lines of C, whatever runs on the Linux install — checks the context: time of day, door state, what the camera sees.
A DO channel fires the siren and the strobe, triggers recording, and pushes an alert over 4G or Ethernet.
Because that logic runs on the box itself, the alarm still works when the internet doesn't. That single property is why alarm duty belongs on a fanless industrial PC rather than a cloud dashboard.
IR beams or door contacts → DI channels
Siren and strobe → DO channels, through a relay module
Logic: DI triggered outside business hours → DO runs the siren for a fixed window, the camera grabs a snapshot, an alert goes out over 4G
If your box has an NPU — the EG828-GL carries 1 TOPS, the EG928A 6 TOPS — gate the siren on the camera actually seeing a person. That one AND-condition deletes most false alarms from animals, wind and falling branches.
The access controller's "door forced" dry contact → DI
The door position contact → a second DI
Logic: door open with no valid credential for more than five seconds → DO fires a local audible alarm, the event is logged, the door camera captures a frame
The same wiring pattern handles fire interlocks: the fire panel's relay → DI → DO releases the magnetic locks on exit doors. Check your local fire code first — many jurisdictions require the egress release path to stay direct and hardware-wired, so let the fanless industrial PC be the logger and notifier, not the safety device.
Water-leak probe, high-temperature contact, smoke relay → DI
DO: shut the valve, start the fan, light the strobe
Scale this up and a weekend project becomes site infrastructure. A site with dozens of doors, beams and sensors needs an I/O-dense box. The EG828-EMS brings 16×DI and 16×DO plus 8×RS485, 2×CAN and 4×Ethernet — effectively a sixteen-zone alarm panel that also speaks Modbus to your PLCs and reports over MQTT. PUSR built it as an energy-monitoring gateway for solar-storage-charging sites, but an alarm concentrator is the same job in a different hat: lots of contacts in, lots of relay outputs out, one Ethernet uplink.
Check dry vs. wet contact.Most alarm accessories are dry contacts. Confirm what your DI channel expects before wiring the whole panel.
Never drive a load straight from DO.A DO channel is a signal, not a power switch. Use it to drive a relay module rated for the siren or lock current.
Common the grounds.Sensor COM, DI common and power ground should share one reference, or you'll chase phantom triggers for a week.
Debounce in software.Contacts bounce. In Node-RED, filter anything shorter than ~50 ms — an RBE or filter node does it.
Share the power rail sensibly.These fanless industrial PCs take wide DC input (typically 9–36 V) with reverse-polarity protection, so one 12 V rail can feed the PC, the sensors and the relay modules.
Test before you trust.Jumper each DI and meter each DO before the first real event. The worst time to discover you needed a Normally-Closed output is during a break-in.
For alarm-plus-automation work, three things matter more than CPU benchmarks:
I/O count and isolation— how many zones in, how many sirens out
Local, editable logic— open Linux with Node-RED beats a closed config tool every time
Survivability— fanless, −25 to 75 °C, dual watchdog, wide-voltage DC, because the alarm box gets mounted where the alarms are
Where the PUSR lineup lands for this job:
EG828-GL— the balanced fanless industrial PC: quad-core RK3568 at 2.0 GHz, a 1 TOPS NPU for video AI, AI/DI/DO plus RS485, CAN and RS232, Ubuntu with Node-RED and Docker pre-installed, optional 4G. Fire and gas safety monitoring is on its application list for a reason. If you want one box handling cameras, siren and doors, start here.
EG928A— the video-heavy option: RK3588J with a 6 TOPS NPU, up to 32 network cameras, face and license-plate recognition, plus DI/DO, CAN and RS485 alongside. For "32 cameras, gate control and alarms" projects, this is the pick.
EG828-EMS— the dense option: 16×DI / 16×DO, 8×RS485, 2×CAN, 4×Ethernet. Buy it when the contact count is high.
EG118— the small open-source option: ESP32 core, 2×DO, 1×DI, 1×AI, RS485, programmable in the Arduino IDE. For one door, one siren and firmware you want to own. Its listed habitat — agricultural irrigation, solar sites, farming, wastewater treatment — tells you where it fits: remote, small, tolerant. The 1×AI channel is a quiet bonus: it reads a 0–10 V level or pressure sensor, which pure-DIO boxes can't.
USR-SH800 panel PC— not an I/O box, but where alarms end up: a wall-mounted touch screen with local SCADA/HMI, RS485 and RS232 to reach your I/O and PLCs, and the WukongEdge engine for logic. A guard who can see, acknowledge and silence an alarm on a screen responds faster than one who gets an email.
How many DI and DO channels, and are they isolated?
Do the DI channels accept dry contacts?
Does the alarm logic run locally when the uplink is down?
Can I reach the I/O from Node-RED or an SDK, or only through a closed tool?
Does the box hold −25 to 75 °C, 9–36 V DC, with a watchdog?
Can it bridge my Modbus and CAN devices into the same logic flows?
The buyer with the ten-port machine will end up with a perfectly decent NAS. But if your project involves a siren, a door strike, a leak probe or a fire-panel relay, buy the fanless industrial PC for its terminal blocks first and its Ethernet ports second.
On day one, wire one DI to one DO through a Node-RED flow. Trigger it, watch it switch, then complicate it — add a time condition, a camera check, a 4G alert. You'll learn more about the machine
