HP3478A Internal USB-GPIB Extension

An internal extension board for the HP3478A Multimeter. USB-CDC-GPIB bridge (Prologix-style) and an internal feature controller.

Uses WCH CH32V203 (RISC-V) MCU.

It enumerates as a standard serial device (e.g., /dev/ttyACM0 or COMx).

PCB here

Menu System

Extended features (Continuity, Temp, etc.) can be accessed using the SRQ button of HP3478A without a PC

1. Enter Menu: Press the Front Panel SRQ.
2. Nav: Press the button again to cycle through modes (M: REL -> M: TEMP -> M: DBM...).
3. Selection (Hover): Stop pressing the button.
   • The display will animate dots (...) after 500ms.
   • After ~2.5 seconds of inactivity, the selected mode is activated.
4. Sub-Menus: Some modes (like Temp) have sub-menus for sensor type and 2W/4W mode. Same cycle logic to select these.
5. Exit: Press the SRQ button to exit back to initial mode (one we entered menu from).

Extended Feature Modes

These can be triggered via the Menu or the serial commands below. Although sub-menu features probably don't work through GPIB commands.
Relative and Statistics depend on the mode you enter the menu from, i.e. if you enter relative from DCV it saves your state and you get relative voltage, from 2W relative 2W etc.

Feature	Command	Description
Continuity	++cont	Fast, latched continuity beeper (2W Ohm, R0: 30 Ohm Range, N3: 3.5 Digits)
Temperature	++temp	Supports PT1000, Type K (via shit CJC... because the AHT20 is near the transformer :)), and Thermistors (Generic 10k/50k/100k & YSI 44000 series)
Relative	++rel	Set NULL/0 offset
dBm	++dbm	Power measurement calculated against 50Ω reference
Diode	++diode	Diode test with 2.5V open circuit voltage and audible latch
Ext Ohms	++xohm	Measures high resistance (>30MΩ) by calculating against internal 10MΩ parallel resistance
Statistics	++math	Cycles display between Live / Min / Max / Avg
Environment	++env	Reads internal AHT20 sensor (Temp/Humidity)
Display	++disp	write custom string to LCD
Normal	++norm	Resets DMM to standard DC Volts state

USB-GPIB Command Reference

Commands start with ++. Any other text is sent directly to the GPIB bus.

Controller Configuration

Command	Arguments	Description
++ver		Get Firmware Version
++help / ++?		List available commands
++rst		Reset the controller
++addr	0-30	Set target GPIB address (Default: 18)
++tmo	ms	Set USB-to-GPIB read timeout (alias: ++read_tmo_ms)

there's also ++savecfg and ++mode to fake prologix compliance

Protocol & Formatting

Command	Arguments	Description
++auto	0-2	0: Off, 1: Read-after-Write, 2: Query-Only (ends in ?)
++eoi	0|1	Assert hardware EOI line on write end
++eos	0-3	Append to Write: 0:CRLF, 1:CR, 2:LF, 3:None
++eor	0-7	Stop Read on: 0:CRLF ... 7:EOI-Only
++eot_enable	0|1	Append character to USB output stream?
++eot_char	dec	The decimal char to append (e.g., 10 for \n)

Bus Management

Command	Description
++read	Read data from target immediately
++write	Write data to target manually
++trg	Issue Group Execute Trigger (GET)
++clr	Selected Device Clear (SDC)
++dcl	Universal Device Clear (resets entire bus)
++spoll	Serial Poll target (returns Status Byte)
++srq	Query SRQ Line status (0=Idle, 1=Active)
++stat	Print controller internal status (Addr, Tmo, etc)

Lower Level Control

Command	Description
++loc	Go To Local (Release Remote Lock)
++gtl	Send "Go To Local" GPIB message
++llo	Local Lockout (Disable front panel buttons)
++ren	Toggle Remote Enable line
++ifc	Interface Clear (Hard Bus Reset)

Known Issues

1. I did NOT use the brain when routing the USBLC6. Either have to DNP the IC and jumper the data lines, or rely on the software hack to check USB SUSPEND to guess if the host is connected. I do the latter, works but meh.
2. The buzzer is not on a hardware TIM pin. It's on a generic GPIO... This means it can't be hardware/timer PWMed. Instead, the firmware is firing a TIM IRQ thousands of times a second just to manually toggle the pin in the ISR..
3. There is a microSD card footprint on the board. The fw does absolutely nothing with it. Data logging?
4. The feature logic is a massive switch.. no VTable, no function pointer structs, it's quite messy