---
title: "NORVI X-DI4 vs X-DI8: Fast Input vs Port Expander"
description: "***Applies to NORVI X-DI4 and NORVI X-DI8*** The X-DI4 and X-DI8 both take external field signals, isolate them, and hand them to the controller as clean digital states. But they're built with different priorities. The X-DI4 gets the signal to…"
url: "https://norvi.io/docs/norvi-x-di4-vs-x-di8/"
updated: "2026-09-21"
category: NORVI X / NORVI X TUTORIALS
---

# NORVI X-DI4 vs X-DI8: Fast Input vs Port Expander

***Applies to NORVI X-DI4 and NORVI X-DI8***

The X-DI4 and X-DI8 both take external field signals, isolate them, and hand them to the controller as clean digital states. But they're built with different priorities. The X-DI4 gets the signal to the CPU as fast and directly as possible, with nothing in between. The X-DI8 trades away some of that speed so you can add far more inputs without using up the controller's limited GPIO pins. Neither is better; they're built for different jobs. The real question is what your application needs: a few inputs that must react in microseconds, or many inputs that just need to report their state reliably.

## The Core Difference in One Sentence

- **X-DI4** wires each input straight to a dedicated microcontroller GPIO through a fast photo-IC optocoupler - built for **speed**.

![](https://norvi.io/wp-content/uploads/2026/09/NORVI-X-DI4-Direct-Gpio.jpg)

- **X-DI8** reads its inputs through an I²C port expander behind a slower phototransistor optocoupler - built for **scale**.

![](https://norvi.io/wp-content/uploads/2026/09/NORVI-X-DI8-I2C.jpg)

## Hardware Architecture

**X-DI4 - Direct GPIO**

| Item | Detail |
| --- | --- |
| Optocoupler | 4 × TLP2361_E (photo-IC output, one channel per chip) |
| Signal path | Optocoupler → directly wired to GPIO |
| GPIO pins used | IN1→IO5, IN2→IO6, IN3→IO7, IN4→IO10 |
| Max inputs | 4 (fixed - one module per controller, since it consumes dedicated pins) |

The TLP2361 isn't a basic optocoupler, it's a smarter chip. Normally, an optocoupler just gives you a weak signal that needs extra parts to turn into a clean digital 0/1. The TLP2361 does that job itself, inside the chip: it takes the light signal and outputs a ready-to-use digital HIGH or LOW directly. So you can wire it straight to a GPIO with no extra circuitry needed which is also why it responds so fast.

**X-DI8 - I²C Port Expander**

| Item | Detail |
| --- | --- |
| Optocoupler | 2 × TLP290-4 (4-channel phototransistor package each → 8 channels total) |
| Signal path | Optocoupler → PCA9538 I²C port expander → SDA/SCL |
| GPIO pins used | Only IO8 (SDA) / IO9 (SCL) - shared bus |
| I²C addresses | 0x70–0x73, set by onboard DIP switches |
| Max inputs | Up to 4 modules stacked on one bus → 32 inputs from 2 pins |

The TLP290-4 is a basic optocoupler, just an LED and a phototransistor, nothing smarter. Its output isn't a clean digital signal, it's just a weak current that changes with light. So before the PCA9538 can read it as a proper 0 or 1, it needs extra parts (pull-up resistors) to shape that current into a real digital signal. The TLP2361 skips this step entirely, which is one reason DI8 ends up slower than DI4.

## Why the Speed Gap Exists (It's Two Compounding Factors)

It's tempting to think DI4 is fast simply because it skips the I²C bus. That's true, but it's only half the reason. The other half is that DI4 and DI8 use different types of optocouplers - and one is a faster class of chip than the other.

| Factor | X-DI4 (TLP2361_E) | X-DI8 (TLP290-4) |
| --- | --- | --- |
| Optocoupler class | Photo-IC - fast, built-in digital output | Phototransistor - slower, needs extra parts to clean up the signal |
| Typical switching time | Data rate 15 MBd; effective board-level switching up to ~5 kHz | ton/toff ≈ 2–3 µs per transition, further limited by I²C polling to ~1 kHz |
| Path to CPU | Direct wire to GPIO - near-zero added latency | Through PCA9538 register → I²C read → CPU - each read is a full bus transaction |

**Bottom line:** DI4 is fast for two reasons: it uses a faster chip, and it skips the bus. DI8 is slower for the same two reasons in reverse: it uses a slower chip, and it goes through the bus. Both effects add up together, so it's not just "wired vs. bus," it's "fast chip and wired" vs. "slow chip and bus."

## Side-by-Side Summary

| Spec | X-DI4 | X-DI8 |
| --- | --- | --- |
| Optocoupler | 4 × TLP2361 (photo-IC) | 2 × TLP290-4 (phototransistor, 4-ch each) |
| Interface | Direct GPIO | I²C via PCA9538 |
| GPIO pins consumed | 4 dedicated pins | 2 shared pins (SDA/SCL) |
| Max channels per controller | 4 (one module only) | 32 (4 modules stacked) |
| Practical switching speed | ~5 kHz | ~1 kHz |
| Isolation voltage | 3750 Vrms | 2500 Vrms |
| Operating temperature | -40 to 125 °C | -55 to 110 °C |
| Best for | PWM signals, encoders, pulse counting, frequency-sensitive inputs | Switch status, sensor states, alarms — anything that doesn't need to be fast but needs to scale |

## Choosing Between X-DI4 and X-DI8

**Pick X-DI4 when:**

- You're reading a pulse train, encoder, flow meter, or PWM signal.
- Timing accuracy matters - you can't afford I²C polling latency.
- You only need 4 or fewer fast inputs.

**Pick X-DI8 when:**

- You're reading simple on/off states - limit switches, door sensors, relay feedback, dry contacts.
- You need more than 4 inputs and want to conserve GPIOs.
- Switching speed in the sub-kHz range is more than fast enough for your process.