B2J0401 MODBUS-RTU Analog & Digital I/O Module
A palm-sized RS-485 slave that senses, counts, times, PWMs and steps — the economical way to put brains at the far end of a low-voltage DC circuit.
- 4 dual-use inputs: digital bit + 16-bit analog register per terminal
- 2 MOSFET outputs: direct, on-delay, PWM, PTO, fast PWM
- Latching emergency-brake register that overrides forced output
- 11 MODBUS function codes, USB factory-reset & firmware port
- 5–24 VDC versions, 55 × 30 mm PCB, DC circuits up to 50 V only
Sensing, Timing and Motion on One Small Board
Machine builders usually solve "read a few sensors, switch a few DC loads, send a pulse train" with three separate cards. The B2J0401 was designed around a dedicated ASIC so that one 55 × 30 mm board covers all three jobs on a MODBUS-RTU / RS-485 network. Each of its four terminals reports both a digital state and a millivolt-resolution analog value, filtered per channel (1–65535 ms) and counted per rising edge — role changes are a register write, not a rewiring job.
On the output side, two open-drain MOSFETs go well beyond on/off: five on-delay trigger modes, standard PWM, PTO pulse-train output for stepper drivers, and a fast-PWM mode on channel 0 for heating control. Three factory output variants (standard, high-current, high-voltage) share identical wiring, and the whole board draws a typical 20 mA.
Two design decisions matter in the field. First, robustness: write-locked configuration registers, a USB recovery port that restores factory communication settings, and EMC test results at double the general industrial EFT requirement. Second, honesty about scope: inputs and outputs are non-isolated and rated for DC circuits up to 50 V only — never switch mains with this board, and never assign it a machine safety function. Safety stops belong to a DQSRN or DA31 safety relay module.
Typical Topology & Wiring Concept
Technical Specifications
| Interface / protocol | RS-485 half-duplex (A/B twisted pair), MODBUS-RTU slave; 11 function codes incl. 0x0F, 0x10, 0x11 (ID 0x2404), 0x16, 0x17 |
|---|---|
| Baud / format | 4800–115200 bps selectable; default 9600 bps, 8 data bits, even parity, 1 stop bit, address 0x02 (range 1–247) |
| Inputs | 4 × dual-use DIN0-3 / AIN0-3; digital threshold ≈ 3 V; operating DC 0–30 V, absolute max DC 50 V |
| Analog conversion | 16-bit register, LSB = 1 mV; effective 8-bit resolution, typical 3 % full-scale accuracy |
| Filtering / counting | Per-channel debounce 1–65535 ms; rising-edge counter to 65535 (rollover), ≤ 5 Hz |
| Outputs | 2 × open-drain MOSFET (DOUT0/1): direct drive, on-delay (5 trigger modes), standard PWM, PTO (ch 0/1), fast PWM (ch 0) |
| Output variants | Standard DC 40 V / 1000 mA · high-current DC 20 V / 2000 mA · high-voltage DC 100 V / 100 mA (max stress 60 V/1.5 A, 30 V/3 A, 150 V/0.2 A) |
| Brake function | BRKCFG selects trigger input, polarity, sensitivity; outputs blocked immediately, BRKFLG latches until cleared over the bus; overrides forced output. Not a certified safety function. |
| Configuration security | SAFELCK write-lock (unlock "UL", auto-relock), MODESAVE persistence ("MS"); USB port for factory reset (within 10 s of power-up) and firmware |
| Supply | 5 / 9 / 12 / 18 / 24 VDC versions (custom on request); ripple < 1 Vp-p; current < 50 mA, typical 20 mA |
| EMC test results | ESD 4 kV (IEC 61000-4-2) and EFT 2 kV/5 kHz (IEC 61000-4-4) with normal operation, tested per IEC 61000-6-2-2016; supply negative must be grounded. "XS" input-reinforced variant available. |
| Mechanical / ambient | 55 × 30 mm PCB (±5 %), holes 49 × 24.5 mm, 5.08 mm pitch, 14 terminals + USB + 4 LEDs; −20 to +60 °C operating, −40 to +85 °C storage |
| Ordering code | B2J04[aa]R[b]T[c]M[dd]; default M31 = 12 V supply, standard MOSFET. Wiring identical across output variants. |
Boundary conditions to respect: no isolation between I/O and supply; DC circuits up to 50 V only; outputs may pulse for ~50 ms during power-up/down — sequence timing-critical loads accordingly.
Typical Applications
Stepper Axis Without a Motion Card
Channel 0 issues the PTO pulse train, channel 1 the direction signal; hard limits trip the brake input, soft limits read back through the spare inputs.
Heating & Power Regulation
Fast PWM on channel 0 modulates heat-seal bars, heater cartridges or LED strings, with an e-stop button latched through the brake register.
Distributed Sensing Node
Four analog channels plus rising-edge counters report level, pressure or flow-meter pulses up a long RS-485 trunk to the plant master.
Small DC Motor & Actuator Control
Direct drive and 5-mode on-delay outputs run pumps, fans and solenoids on 5–48 V DC rails, sized by the three MOSFET output variants.
Frequently Asked Questions
What kind of device is the B2J0401 — a relay board or an I/O module?
It is a solid-state I/O module, not a contact relay board. The two outputs are open-drain MOSFETs that can switch, time-delay, PWM-modulate or pulse-train (PTO) low-voltage DC loads, while the four input terminals each read either a digital contact or a 16-bit analog voltage register. If you need a volt-free relay contact or must switch AC mains, choose the relay-isolated B2J0302 instead.
How do the dual-use inputs behave on the same terminal?
Each of the four terminals is simultaneously mapped to a digital status bit (threshold about 3 V) and an analog register with 1 mV resolution (typical accuracy 3 % of full scale, effective 8-bit). Every channel also has its own 1–65535 ms debounce filter and a rising-edge counter that rolls over at 65535 for pulse rates up to 5 Hz — so a float switch, a potentiometer and a flow meter can share the same board.
Can it really drive a stepper motor without a separate motion controller?
For simple point-to-point moves, yes: output channel 0 generates the PTO step pulses and channel 1 sets direction, straight into a stepper driver's PUL/DIR inputs. Complex interpolation still belongs in a real motion controller, but for feeders, indexers and single-axis positioning the on-board PTO removes an entire module from the panel.
What does the emergency-brake function do, and is it a certified safety function?
The BRKCFG register nominates one input as a brake trigger with configurable polarity and sensitivity. When tripped, both outputs are blocked at once, the BRKFLG flag latches to 'BK' until cleared over the bus, and the brake outranks any forced-output command — a crashed master cannot restart the load. It is, however, a configurable logic feature with no Type/SIL/PL rating: for machine safety stops, use a DQSRN or DA31 safety relay module, never this board.
What happens if I set a wrong baud rate and lose contact with the module?
Recovery is built in. Connect the on-board USB port within about 10 seconds of power-up and the host software restores the factory defaults — 9600 bps, 8 data bits, even parity, 1 stop bit, address 0x02 — and can load new firmware. Sensitive registers are also write-locked (unlock code 'UL', auto-relock after every write or CRC error), so bus noise cannot silently rewrite the configuration.
Which output variant and supply voltage should I order?
Pick the output stage by load: standard MOSFET for DC 40 V / 1000 mA general loads, high-current for DC 20 V / 2000 mA, high-voltage for DC 100 V / 100 mA — wiring is identical across the three. Supply versions cover 5, 9, 12, 18 and 24 VDC (custom voltages on request), drawing under 50 mA. Remember the absolute limit: DC circuits up to 50 V only, never mains.
Do you support OEM builds, and what are the MOQ and lead time?
Yes — for volume orders the communication parameters can be factory-fixed, and unused terminals, buttons or LEDs deleted to trim unit cost, with an input-reinforced 'XS' variant for electrically harsh cabinets. Sample quantities are available for evaluation; MOQ and current lead time depend on the variant mix, so include them in your enquiry.