RS485 Non-Isolated Breakout Board

Revision Status

Revision	Status	Changes
Rev A	Active	(Initial release)

The RS485 Non-Isolated Breakout Board is a clean, compact differential serial interface designed purely for half-duplex communication. Derived from the earlier RS485 + 48 V Passthrough Board, it removes the high-voltage power routing to eliminate potential noise coupling and high-voltage risks near logic lines.

Features

• Transceiver: Based on the SP3485EN 3.3 V half-duplex RS485 transceiver.
• Compact Footprint: Uses smaller 2.54 mm pitch screw terminals and 0603/0402 passive component footprints to reduce overall board size.
• Direction Control: A pin jumper links the Receiver Enable (/RE) and Driver Enable (DE) lines, allowing a single microcontroller GPIO to control the half-duplex direction state.
• Robust Termination: Features a jumper-selectable 120 Ω termination resistor in a conservative 1206 package to comfortably handle common-mode voltages (commonly -7 V to +12 V).
• Improved Biasing: Jumper-selectable bus biasing options configured with 470 Ω and 4.7 kΩ resistors to establish a stable bus idle state.
• Brighter Indicators: RX/TX LED indicator resistors reduced from 2.2 kΩ to 470 Ω for better visibility. Solder jumpers can be cut if low-power mode is required.
• ESD Protection: Built-in TVS diode array on the differential A/B lines to protect the transceiver from electrostatic discharge and line transients.

Hardware Configuration & Pinout

Microcontroller Interface (Logic Side)

Pin	Label	Description
1	RO	Receiver Output (UART RX)
2	DI	Driver Input (UART TX)
3	DE	Driver Enable (Active High)
4	/RE	Receiver Enable (Active Low, usually tied to DE)
5	3.3V	3.3 V Logic Supply
6	GND	Common Ground

Bus Side Interface (Screw Terminals)

Two terminal blocks are provided that are electrically connected to the same lines, allowing for easy daisy-chaining of the bus.

Pin	Terminal	Description
1	D1/B	RS485 Inverting Line
2	D0/A	RS485 Non-inverting Line
3	GND	Bus Ground Reference

Schematic

Bill of Materials (BOM)

Reference	Qty	Value	Footprint	LCSC Part No	MFR Part No	Description
C1	1	100nF	Capacitor_SMD:C_0805_2012Metric	C49678	CC0805KRX7R9BB104	Unpolarized capacitor, small symbol
D1,D2,D7	3	SML-D12M8WT86	LED_SMD:LED_0603_1608Metric	C510025	SML-D12M8WT86	Light emitting diode
D4	1	SM712_SOT23	Package_TO_SOT_SMD:SOT-23	C7420375	SM712	7V/12V, 600W Asymmetrical TVS Diode Array, SOT-23
J2,J3	2	Screw_Terminal_01x03	TerminalBlock_Phoenix:TerminalBlock_Phoenix_MPT-0,5-3-2.54_1x03_P2.54mm_Horizontal	C5288763	1725669	Generic screw terminal, single row, 01x03, script generated (kicad-library-utils/schlib/autogen/connector/)
JP1,JP7	2	Jumper_2_Small_Open	Connector_PinHeader_2.54mm:PinHeader_1x02_P2.54mm_Vertical	C42431798	PH2.54-1X2P-H25	Jumper, 2-pole, small symbol, open
JP5,JP6	2	Jumper_3_Open	Connector_PinHeader_2.54mm:PinHeader_1x03_P2.54mm_Vertical	C42431836	PH2.54-1X3P-H25	Jumper, 3-pole, both open
Q1	1	AO3401A	Package_TO_SOT_SMD:SOT-23	C15127	AO3401A	-4.0A Id, -30V Vds, P-Channel MOSFET, SOT-23
R1,R2,R3,R4	4	100R	Resistor_SMD:R_0603_1608Metric	C22775	0603WAF1000T5E	Resistor
R5,R6	2	10K	Resistor_SMD:R_0402_1005Metric	C25744	0402WGF1002TCE	Resistor
R7,R9	2	4K7	Resistor_SMD:R_0603_1608Metric	C23162	0603WAF4701T5E	Resistor
R8	1	120R	Resistor_SMD:R_1206_3216Metric	C17909	1206WAF1200T5E	Resistor
R10,R11	2	22R	Resistor_SMD:R_0603_1608Metric_Pad0.98x0.95mm_HandSolder	C23345	0603WAF220JT5E	Resistor
R12,R13,R18	3	470R	Resistor_SMD:R_0402_1005Metric	C25117	0402WGF4700TCE	Resistor
R15,R16	2	470R	Resistor_SMD:R_0603_1608Metric	C23179	0603WAF4700T5E	Resistor
U1	1	SP3485EN-L/TR	Package_SO:SOIC-8_3.9x4.9mm_P1.27mm	C8963	SP3485EN-L/TR	Industrial 3.3V Low Power Half-Duplex RS-485 Transceiver 10Mbps, SOIC-8

Test Verification & Code

This board has been tested successfully at baud rates ranging from 300 baud up to 921,600 baud over 20 meters of Cat 6 cable using Raspberry Pi Pico 2 W nodes.

MicroPython Transmit Example

This script runs on the transmitting node, sending a counter value every second.

from machine import UART, Pin
import time

# Configure UART
uart = UART(0, baudrate=921600, tx=Pin(0), rx=Pin(1))

# Define the Driver Enable / Receiver Enable pin
de_re = Pin(2, Pin.OUT)

counter = 0

while True:
    # Enable transmit mode
    de_re.value(1)

    message = "Okay here's a counter value: " + str(counter)
    uart.write(message.encode())
    print("Sent:", message.strip())

    uart.flush()
    time.sleep(1)

    if counter >= 10:
        counter = 0
    else:
        counter += 1

MicroPython Receive Example

This script runs on the receiving node, reading the differential signal and decoding it.

from machine import UART, Pin
import time

# Configure UART
uart = UART(0, baudrate=921600, tx=Pin(0), rx=Pin(1))

# Define the Driver Enable / Receiver Enable pin
de_re = Pin(2, Pin.OUT)

# Set board to receive mode
de_re.value(0)

print("Waiting for data...")

while True:
    if uart.any():
        data = uart.read()
        try:
            decoded_text = data.decode("utf-8")
            print("Received Raw:", data)
            print("Decoded Text:", decoded_text.strip())
        except UnicodeError:
            print("Received data, but could not decode as UTF-8.")

    # Small delay to avoid hogging the CPU
    time.sleep(0.1)

Project Files

• RS485 Non-Isolated Breakout Board KiCad Project (ZIP)