FS9721-BLE Wireless Multimeter Telemetry

The Fortune Semiconductor FS9721-LP3 (along with variants such as the FS9711 and FS9922) is one of the most widely deployed 4000-count DMM controllers on the planet, found in multimeters from Mastech, UNI-T, Voltcraft, TekPower, and Velleman. However, legacy PC-link solutions require optically isolated USB cables, tethering the instrument to a workbench and risking ground loop issues.

FS9721-BLE solves this by retrofitting handheld multimeters with an internal Nordic nRF52832 BLE bridge, creating an end-to-end, ultra-low-power wireless telemetry pipeline directly paired to a custom native macOS companion workstation written in Swift 6 and SwiftUI.

Hardware Retrofit & Macro Photography

Instrument Shell: Internal installation within a Mastech MS8217 multimeter.

Micro-soldering: Tapping Pin 64 (TXD) and Pin 84 (ENTX) on the 100-pin QFP.

Wireless Core: Nordic nRF52 module mounted on the internal shielding wall.

Key Engineering Innovations

• True Galvanic Air-Gap Isolation: Replaces wired PC cables with Bluetooth Low Energy, preserving the meter's > 1000V CAT III / CAT IV safety isolation without hazardous ground loops.
• Dynamic Active-Low Hardware Gating (ENTX): Directly controls the DMM controller's internal UART clock generator on Pin 84. When disconnected, the serializer is held in deep standby, achieving near-zero idle parasitic draw on the 9V battery.
• Aggressive Embedded Power Budgeting:
  • Internal DC/DC step-down buck converter active (sd_power_dcdc_mode_set).
  • Hardware 32.768 kHz external crystal (LFXO) eliminates 16 MHz HFCLK periodic calibration cycles.
  • All 30 uncommitted GPIOs placed in high-impedance INPUT_Disconnect with pull resistors disabled.
  • BLE connection slave latency ($L=3$) significantly reduces radio duty cycle during telemetry streaming.
• Zero-Allocation 14-Byte Frame Synchronizer: Monotonic framing engine verifies high-nibble sequence bytes (0x1_ through 0xE_), rejecting line noise, incomplete packets, and break conditions.
• Full 7-Segment Glyph Reconstruction: Decodes raw LCD segment bitmasks into numerical values, polarity signs, metric SI multipliers (nano, micro, milli, kilo, Mega), decimal points, and active flags (AC/DC, Auto, Hold, Relative, Continuity, Diode, Temperature).
• Native macOS Companion Workstation (Swift 6 & SwiftUI):
  • Hardware-accelerated SwiftUI Canvas d'Arsonval analog meter with mechanical ballistic damping.
  • Live statistics calculating running Mean, Min, Max, Peak-to-Peak ($\Delta$), Sample Count, and live sampling frequency (Hz).
  • RFC 4180 compliant CSV logging with millisecond timestamps for lab data acquisition.
  • Offline Mock Simulator switch for instant UI testing without physical hardware.

Pinout & Wiring Architecture

Compatible Multimeter Models

Because the Fortune FS9721-LP3 is an industry-standard controller, this firmware and companion app work out of the box with meters compatible with the libsigrok fs9721 driver family:

• Mastech: MS8217, MS8218, MS8229, MS8209, MS8250B
• UNI-T: UT60A, UT60E, UT60G, UT61B, UT61C, UT61D
• Voltcraft: VC-820, VC-830, VC-840, VC-850
• TekPower: TP4000ZC, TP8268, TP9605BT
• PeakTech & V&A: PeakTech 3330, VA18B, Digitek DT-4000ZC

RFC 4180 Lab Telemetry CSV Format

Data recorded by the macOS workstation is formatted for immediate import into Python (pandas/NumPy), MATLAB, and Excel:

# MS8217 Wireless Telemetry Log
# Host: macOS Sonoma | App Version: 2.0.0
# Interface: Nordic UART Service (BLE) | Peripheral: MS8217-BLE
# Session Start: 2026-11-06T02:40:00.000+05:30
Timestamp_ISO,Elapsed_Sec,Value_Displayed,Value_SI,Unit,Prefix,Mode,Flags,Raw_Hex
2026-11-06T02:40:00.000+05:30,0.000,-0900,-0.900,%,m,AC,AUTO,152f3d435f677d8f9da0b8c0d4e8
2026-11-06T02:40:00.350+05:30,0.350,-0889,-0.889,%,m,AC,AUTO,152f3d435f677d8f9da0b8c0d4e8