E04 Modbus I/O Module — Datasheet & Manual

Datasheet & User Manual · Board owledge (MODEL E04SD) · Document rev 1.0 · 2026-06-18

1. Overview

The E04 Modbus I/O Module is an ESP32-based industrial I/O device with 4 digital inputs and 4 digital outputs. It is exposed simultaneously over Modbus TCP (Ethernet) and Modbus RTU (RS-485), sharing one register map, and is discoverable on the LAN via mDNS / Zeroconf. A supervisory platform browses mDNS, reads capability metadata from TXT records, then polls/controls the device — no static IP or manual register-map documentation required for integration.

The E04SD is the all-digital sibling of the E02 module (2 analog/digital in, 1 PWM out). Both run the same firmware, selected at build time by a Kconfig board choice.

2. Key Features

I/O

4× digital inputs (debounced)
4× digital outputs (on/off)
Single-colour status LED

Connectivity

Modbus TCP over Ethernet, port 502
Modbus RTU over RS-485 (concurrent)
mDNS service advertisement
DHCP client (static IP optional)

Configuration

All params via Modbus registers
NVS persistence (survives reboot)
SoftAP web portal for provisioning

Safety

Per-output fail-safe state (bitmask) on link/master loss
All outputs OFF at boot until first write

3. Specifications

Parameter	Value
MCU	ESP32-WROOM-32U V2.1 (Xtensa dual-core, no PSRAM)
Flash	4 MB (DIO, 40 MHz)
Firmware	e04_modbus v1.0.0, built on ESP-IDF 5.4
Inputs	4 × digital, debounced (3 consistent reads @ 10 ms)
Outputs	4 × digital (on/off, push-pull GPIO)
Ethernet	10 Mbps, full-duplex
Serial	RS-485 half-duplex, auto-direction transceiver (no DE/RE pin)
Protocols	Modbus TCP (port 502) + Modbus RTU, concurrent; mDNS
Addressing	DHCP client; static IP fallback configurable

4. Pinout

4.1 Board I/O

ESP32 GPIO	Function	Notes
12	Status LED	single colour; strapping pin (MTDI) — driven after boot
14	Config-mode button	hold LOW at boot → provisioning
39	Digital input 1	input-only pin (external pull on board)
36	Digital input 2	input-only pin (external pull on board)
35	Digital input 3	input-only pin (external pull on board)
34	Digital input 4	input-only pin (external pull on board)
25	Digital output 1
26	Digital output 2
27	Digital output 3
4	Digital output 4

Inputs are on GPIO 34–39, which are input-only and have no internal pull-up/down. The board must provide external pull resistors on each input line.

4.2 Ethernet — ENC28J60 (SPI)

ESP32 GPIO	ENC28J60
5	CS
18	SCLK
19	MISO
23	MOSI
N/A	INT (not connected — driver runs in polling mode)

4.3 RS-485 — Modbus RTU (UART2)

ESP32 GPIO	Function
17	UART2 TX → RS-485 transceiver
16	UART2 RX ← RS-485 transceiver
N/A	DE/RE — none (auto-direction transceiver)

5. I/O Channels

5.1 Inputs

All 4 inputs are digital. Each is debounced (3 consistent reads at a 10 ms poll interval) and reported over Modbus as 0 / 1 in its input register.

5.2 Outputs

Each of the 4 outputs is driven from its output register: any non-zero value drives the GPIO high (ON), zero drives it low (OFF). Writes take effect immediately. At boot and during fail-safe the outputs are forced to a defined state.

6. Network & Discovery

DHCP client by default; static IP configurable via provisioning.
Hostname: e02-<wifi_mac_no_colon>.local (shared device-family prefix; the model TXT distinguishes E04)
mDNS service: edge_modbus._tcp.local, port 502

TXT records (capability advertisement):

Key	Example	Meaning
`model`	e04_modbus	Device model
`fw`	1.0.0	Firmware version
`i`	4	Input channels
`o`	4	Output channels
`unit`	1	Modbus unit/slave ID
`proto`	modbus-tcp	Protocol identifier
`mac`	B4E62DDE225D	WiFi MAC (no colons)

7. Modbus Interface

Parameter	TCP	RTU
Role	Slave / Server (both run concurrently, shared registers)
Transport	Ethernet, port 502	RS-485, UART2
Address	Unit ID (MBAP)	Slave address = Unit ID register
Default line	—	9600 baud, 8N1 (configurable)
Function codes	03 (read holding), 06 (write single holding)
Byte order	Big-endian (standard Modbus)

RTU baud / parity / stop-bits and the unit/slave id take effect after a reboot (the Modbus controllers are created once at startup).

8. Register Map (Holding Registers)

Read with FC 03, write with FC 06. Layout: configuration block first, then live I/O.

Addr	Register	Access	Persist	Notes
0	Unit ID / RTU address	R/W	NVS	1–247; effective after reboot
1	Fail-safe output bitmask	R/W	NVS	bits 0–3 → outputs 1–4
2	Fail-safe timeout (s)	R/W	NVS	0 = disabled
3	RTU enable	R/W	NVS	0/1; effective after reboot
4	RTU baud / 100	R/W	NVS	96=9600 … 1152=115200
5	RTU parity	R/W	NVS	0=none, 1=odd, 2=even
6	RTU stop bits	R/W	NVS	1 or 2
7	Input 1 value	R	—	0 / 1 (debounced)
8	Input 2 value	R	—	0 / 1
9	Input 3 value	R	—	0 / 1
10	Input 4 value	R	—	0 / 1
11	Output 1	R/W	—	0 = OFF, non-zero = ON
12	Output 2	R/W	—	0 / 1
13	Output 3	R/W	—	0 / 1
14	Output 4	R/W	—	0 / 1
15	Status flags	R	—	bitfield (below)

Status flag bits (register 15)

Bit	Mask	Meaning
0	0x01	Ethernet link / IP up
1	0x02	Modbus master polled recently (any transport)
4	0x10	Outputs forced to fail-safe
5	0x20	RTU transport enabled

Bits 2–3 (wire-break) are defined in the shared register layout but are only meaningful on the E02 (4–20 mA) variant; they read 0 on the E04.

9. Status LED (GPIO 12)

State	LED	Meaning
Provisioning	Fast blink	Config mode (SoftAP portal active)
Initializing	Slow blink	Booting, network not ready
Running	Solid ON	Normal operation (Modbus serving)

10. Fail-safe & Boot

Boot: all outputs initialised OFF until the first valid write.
Fail-safe trigger: Ethernet link down, or no Modbus master access within the fail-safe timeout (register 2, if > 0).
Fail-safe action: each output driven to its bit in the fail-safe bitmask (register 1).

Boot sequence: init GPIO + safe outputs (LED slow-blink) → bring up Ethernet, acquire DHCP → start mDNS + Modbus TCP/RTU slaves (LED solid) → enter input-poll loop.

11. Configuration (Manual)

11.1 Via Modbus

Write configuration registers (0–6) with FC 06. Config values persist to NVS. Transport/serial params (unit id, RTU baud/parity/stop/enable) apply on the next reboot; fail-safe bitmask/timeout and the outputs apply live.

11.2 Via SoftAP web portal (provisioning)

Hold GPIO 14 LOW at power-on/reset — the LED blinks fast.
Join WiFi AP E02-xxxxxx (password edge12345).
Open http://192.168.4.1/ and set unit id, fail-safe (outputs bitmask / timeout), RS-485 (enable/baud/parity/stop) and optional static IP.
Save → settings persist to NVS and the device reboots into normal operation.

Security: Modbus is unauthenticated (per the standard). Deploy only on trusted networks. Change the default SoftAP password per site policy.

12. Build & Flash

Toolchain: ESP-IDF v5.4, target esp32. The E04SD variant is a compile-time Kconfig choice (CONFIG_BOARD_E04SD=y, in sdkconfig.e04sd). Managed components (esp-modbus v2.x, mdns, enc28j60) download on first build.

idf.py set-target esp32
idf.py -B build_e04 -DSDKCONFIG=sdkconfig.e04 \
       -DSDKCONFIG_DEFAULTS="sdkconfig.defaults;sdkconfig.e04sd" build
idf.py -B build_e04 -p /dev/ttyUSB0 flash monitor

Binary ~979 KB; factory partition 3.88 MB (4 MB flash, no OTA).

13. Troubleshooting

Symptom	Check
No IP / not discoverable	Ethernet cable & ENC28J60 wiring (CS5/SCLK18/MISO19/MOSI23); DHCP server present; LED stuck slow-blink = no IP yet
TCP master can't connect	Confirm IP (serial log), port 502 open, only the needed master polling
RTU no response	RTU enabled (reg 3); matching baud/parity/stop & slave address; A/B lines & ground; UART2 = GPIO17/16
Input always 0 or 1	GPIO 34–39 need external pull resistors; verify field wiring & signal level
Output doesn't switch	Write to registers 11–14; check fail-safe isn't active (status bit 4)
Can't enter config mode	GPIO14 must read LOW at boot; LED should switch to fast blink

E04 Modbus I/O Module — Datasheet & Manual · Technimal · generated from firmware modbus_2_io (config.h / reg_map.h, CONFIG_BOARD_E04SD), build verified on ESP-IDF 5.4.