E02 Modbus I/O Module — Datasheet & Manual

Datasheet & User Manual · Board owledge (MODEL E02SA) · Document rev 1.0 · 2026-06-18

1. Overview

The E02 Modbus I/O Module is an ESP32-based industrial I/O device with 2 configurable analog/digital inputs and 1 PWM output. 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.

2. Key Features

I/O

2× inputs, each: Digital, 4–20 mA, or 0–10 V
1× output via PWM (0–100 %)
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

Configurable fail-safe output on link/master loss
Output OFF at boot until first write
4–20 mA wire-break detection

3. Specifications

Parameter	Value
MCU	ESP32-WROOM-32U V2.1 (Xtensa dual-core, no PSRAM)
Flash	4 MB (DIO, 40 MHz)
Firmware	e02_modbus v1.0.0, built on ESP-IDF 5.4
Inputs	2 × (Digital / 4–20 mA / 0–10 V), software-selectable per channel
Output	1 × PWM, 1 kHz, 12-bit duty, 0–100 %
Ethernet	10 Mbps, full-duplex
Serial	RS-485 half-duplex, auto-direction transceiver (no DE/RE pin)
ADC	12-bit, ADC1, 12 dB attenuation, 16-sample averaging
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
25	Digital input 1
26	Digital input 2
36	Voltage input 1	0–10 V (ADC1_CH0)
34	Voltage input 2	0–10 V (ADC1_CH6)
39	Current input 1	4–20 mA (ADC1_CH3)
35	Current input 2	4–20 mA (ADC1_CH7)
4	Output	PWM (LEDC)

Each logical input has 3 physical pins (digital / voltage / current); the active pin is chosen by the channel's mode register.

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

Each input's mode is set by its mode register (0=Digital, 1=4–20 mA, 2=0–10 V). Values are reported over Modbus in engineering units:

Mode	Reported value	Example
Digital	`0` / `1` (debounced, 3 consistent reads @ 10 ms)	1 = high
4–20 mA	centi-mA (mA × 100)	`2000` = 20.00 mA
0–10 V	centi-volt (V × 100)	`1000` = 10.00 V

In 4–20 mA mode a reading below ~3.5 mA sets the wire-break status flag.

5.2 Output

The output register holds a duty percentage 0–100, driven on a 1 kHz PWM. Writes take effect immediately. At boot and during fail-safe the output is forced to a defined value (default 0 %).

6. Network & Discovery

DHCP client by default; static IP configurable via provisioning.
Hostname: e02-<wifi_mac_no_colon>.local
mDNS service: edge_modbus._tcp.local, port 502

TXT records (capability advertisement):

Key	Example	Meaning
`model`	e02_modbus	Device model
`fw`	1.0.0	Firmware version
`i`	2	Input channels
`o`	1	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	Input 1 mode	R/W	NVS	0=Dig, 1=4–20 mA, 2=0–10 V
2	Input 2 mode	R/W	NVS	same
3	Fail-safe output value	R/W	NVS	percent 0–100
4	Fail-safe timeout (s)	R/W	NVS	0 = disabled
5	RTU enable	R/W	NVS	0/1; effective after reboot
6	RTU baud / 100	R/W	NVS	96=9600 … 1152=115200
7	RTU parity	R/W	NVS	0=none, 1=odd, 2=even
8	RTU stop bits	R/W	NVS	1 or 2
9	Input 1 value	R	—	scaled (see §5.1)
10	Input 2 value	R	—	scaled (see §5.1)
11	Output	R/W	—	PWM percent 0–100
12	Status flags	R	—	bitfield (below)

Status flag bits (register 12)

Bit	Mask	Meaning
0	0x01	Ethernet link / IP up
1	0x02	Modbus master polled recently (any transport)
2	0x04	Input 1 wire-break (4–20 mA < 3.5 mA)
3	0x08	Input 2 wire-break
4	0x10	Output forced to fail-safe
5	0x20	RTU transport enabled

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: output initialised OFF (0 %) until the first valid write.
Fail-safe trigger: Ethernet link down, or no Modbus master access within the fail-safe timeout (register 4, if > 0).
Fail-safe action: output driven to the fail-safe value (register 3).

Boot sequence: init GPIO + safe output (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 (0–8) with FC 06. Config values persist to NVS. Transport/serial params (unit id, RTU baud/parity/stop/enable) apply on the next reboot; input modes, output and fail-safe values 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, input modes, fail-safe, 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. Managed components (esp-modbus v2.x, mdns, enc28j60) download on first build.

idf.py set-target esp32
idf.py build
idf.py -p /dev/ttyUSB0 flash monitor

Binary ~988 KB; factory partition 3.88 MB (4 MB flash, no OTA).

13. Calibration

The analog scaling constants in main/config.h are placeholders and must be calibrated per hardware revision before field use:

Constant	Meaning
`ADC_MV_AT_FULL_VOLT`	ADC millivolts when the 0–10 V terminal reads 10.00 V (divider ratio)
`CURRENT_SENSE_OHMS`	RS sense resistor: I(mA) = V_adc(mV) / R
`CURRENT_WIRE_BREAK_CMA`	wire-break threshold in centi-mA (default 350 = 3.5 mA)

Recommended: 2-point calibration per mode (e.g. apply 4 mA & 20 mA, 0 V & 10 V) and adjust the constants so reported values match a reference meter.

14. 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 5); matching baud/parity/stop & slave address; A/B lines & ground; UART2 = GPIO17/16
Analog values wrong	Calibrate `ADC_MV_AT_FULL_VOLT` / `CURRENT_SENSE_OHMS`; verify input mode register
Output only 0 % / 100 %	Write percent 0–100 to register 11; 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

E02 Modbus I/O Module — Datasheet & Manual · Technimal · generated from firmware modbus_2_io (config.h / reg_map.h), build verified on ESP-IDF 5.4.