Protek 506 Multimeter (Also Known as Voltcraft VC506) Serial to Bluetooth Interface For PC

Motivation:

While i was browsing internet flea markets, stumbled upon a PROTEK 506 Multimeter which was reasonable price, purchased and repaired it. While repairing and using it i decided to design a wireless (bluetooh) interface for onboard RS232 port. It belongs to 90's according to IC's on DMM mainboard but it is still a good multimeter with unique diode function and PC connection. As you can see my multimeter has had a very tough life before...

You are here because most likely you purchased this multimeter and already know that specs and functions but anyway here are some general specifications:

• Number of digits 3¾
• Number of counts 4000
• True RMS AC, AC & DC
• DC Voltage Ranges 400mV to 1000V
• AC Voltage Ranges 400mV to 750V
• Resistance Ranges 400 ohm to 40 Mohm
• Max. current 20A
• Frequency Ranges 10Hz to 10MHz
• Inductance Range 100H
• Capacitance Ranges 100μF
• RS-232 Interface
• Logic Function
• Diode Mode Test Volt 3.3V Max
• Signal out 2048Hz, 4096Hz, 8192Hz Square wave
• Power 9V battery (I strongly recommend 3.7v Li-ion battery with boost converter)
• Retail Price in 2002 169$

There are some videos on YouTube about this multimeter, go check them to get an idea.

PC connection with RS-232 cable works fine and provides "some" galvanic isolation with two photocouplers (PC817) but i didn't want anything connected to PC ground (which can lead short circuits like oscilloscopes) and cable connections already messy in my workbench. Also most of modern PC and laptop models no longer have RS-232 ports.

So i decided to make it wireless...

This guide can be used for many multimeters with serial port (RS-232) support.

Reverse Engineering and Prototyping:

Lets get some technical details about that RS-232 port before doing anything. In operating manual there is a section about how to interface multimeter serial port with PC.

We are interested about cable pin connection and communication Spec.

• Baud Rate: 1200 bps
• Data Bit: 7
• Stop Bit: 2
• Parity: None

1200 bps baud rate is very slow on today standarts which is a problem for us because most of wireless or bluetooh serial chips are way faster than 1200 bps, most of them support 9600bps at minimum. In my design route i wondered about a MCU to get serial data, read it and transmit data via wireless or bluetooth but this may cause unwanted delays on measurement, power consumption and software is not my strong suit either. That MCU idea was a dead end. Issue

While i was browsing serial to bluetooth converters i decided to look up their main chipset to see if some of them support lower baud rates. CSR-BC417 seemed very promising and it is widely available as you might already know as HC-06.

BC417 Datasheet UART Interface Settings:

Since i am a cheap b@stard i looked up my personal scrapyard to find anything usable for testing and prototype. Years ago "before GPS recievers are embedded in our phones" i used to have a Royaltek Bluetooth 2100 external bluetooh for iGO navigation for my nokia 6630. On that device scrap motherboard i found MB-C041 bluetooth module and desoldered it from main PCB. Tested module on 9600 default baud with putty and it worked fine but there are two problems;

First one is MB-C041 parameters cannot be configured like HC-06 so i looked after changing device firmware and found out that firmware parameters can be altered via CSR's Blue Suite program. Interfacing MB-C041 with PC wasn't easy but amazingly some talanted people wrote a software library for it. CSR-SPI-FTDI I will not get into details in here but there are also some videos on YouTube about this firmware topic. To briefly summarize it you have to do below steps to connect MB-C041(or BC417 based module):

• Grab a FTDI FT232R USB to UART converter chip (I used BOB-00718 from sparkfun)

• Use above library with Zadig

• Connect SPI_CS# ,SPI_CLK, SPI_MOSI,SPI_MISO, GND and 3.3V to module.(Library readme have connection diagram)

• Check connectivity via -BlueFlashCmd.exe identify- it should look like this:

  

• Make backup of existing firmware via "BlueFlash" before doing ANYTHING:

Then you have 2 options: First one is easy, both are include small portion of risk of course:

1. Use my ready to flash Firmware which already have correct parameters for PROTEK 506 but I don't have any responsibility if you brick your module :rage1: Just keep in mind that this firmware is for BC3 core and you have to modify and merge your 3 firmware parameters which are PSKEY_BDADDR, PSKEY_ANA_FTRIM and PSKEY_ANA_FREQ from your original firmware with update psr.I will not get here with details. You can learn from here if you wish.)

2. Use PSTool (Bluecore Persistent Store) to edit following firmware parameters:

   • Find PSKEY_UART_BAUD_RATE (0x204) in PSTool List

   • Change Value to 0x0005 hex or 5 dec.

     

   • Save and remove SPI pins

There is also way easier method to change device name and baud settings which is with AT commands with arduino if you don't have CSR-SPI-FTDI but i cannot guarantee this method. There is a tutorial how to do so.

The second problem is PROTEK 506 normally works on RS-232 levels which is about +/-15v but our MB-C041 (BC417) works on 3.3v logic levels which is not OK for HC-06. This issue can be solved with two ways:

1. Use MAX232 to reduce logic levels to 5V then use a level shifter with BSS138 to convert 5V logic to 3.3V. For prototype i chosed this way.
2. Use MAX3232 (Notice extra "3") to reduce RS-232 levels directly to 3.3V.

Both methods will work fine, since i needed quick prototype i chosed first method.

Removed MAX232 from an old satellite reciever with it passive components, (Remember, i said cheap b@stard.) added 2 regulators which are 5 and 3.3 volts, wired up MB-C041 and level shifter and cobbled up this concoction to a perfboard:

I even go further and draw a schematic in KiCAD:

This prototype was 20*40mm so i had to relocate buzzer.

For initiating serial connection DTR must be HIGH, RTS must be LOW. If you look up Sigrok Multimeter IC's Metex 14-byte ASCII section you can clearly see that statement: The serial port settings can vary from device to device, e.g. 600/1200 baud, 7n2, RTS low, DTR high. Protek 506 have Samsung KS57C2016 MCU. There are similiar multimeters such as Voltcraft VC506, Metex M3610D, RadioShack 22-168 etc. with same MCU so i believe this design can be suitable with such multimeters. But keep in mind that this schematic is valid for PD506(MAIN) VER.02 revision, i have seen different schematics and diffirent PCB revisions online so before connecting this module to your multimeter be advised and measure pinouts.

I'm a person who like to modify devices but keep it original as possible while modding it so i didn't want to bypass photocouplers and make a direct connection with multimeters MCU.(Pin 48 and Pin 49 IIRC) This approach is dangerous for MCU if something went wrong on bluetooth module side. You can still find MCU chip even if it is an obsolete part but i think that compiled MCU software is unobtanium at this point. If you know how to make a backup/dump of that MCU software please let me know. BTW you can still use RS-232 cable connection with this mod so no loss of function.

Here is connection points on multimeter pcb:

Yellow (RX) Cyan (TX), Black and Gray (GND), Blue (+9V), Red (+5V)

Hardware Release:

After poorly botched prototype, i decided to design a proper PCB for it.

There is nothing special in design different than prototype. Instead of using level shifter, MAX232 and MB-C041 combo, i used MAX3232 from Texas Instruments and HC-06. HC-05 may also compatible but never tested by me.

• R3 and D2 LED may not be populated by user to reduce power consumption however i like to see activity on connection.

• Programming/Debug Port: TP1-CSB TP2-CLK TP3-MOSI TP4-MISO

• Power Pin: TP5-3.3V

• 3.3v Level Serial Monitor Port: TP7-RX TP8-TX

• JP1-Sets PIO3 to HIGH if bridged. For AT mode but not tested yet with actual hardware.

• -J3- JUMPER

• 1-Gray (GND)

• 2-Blue (+9V)

• 3-Red (+5V)

• 4-Cyan (TX)

• 5-Black(GND)

• 6-Yellow (RX)

• Schematic have LCSC codes for parts, except HC-06 module.

Here is the finished and assembled PCB:

Installed Module:

Software:

Vendor Software

Original Software for PROTEK 506 can be found on this 2006 web archive links:

Windows

MS-DOS

Also there is a diskette image from 2022:

Diskette Image

PC software is designed for MS-DOS 5.0 or Higher, Windows 3.1 or Higher, Windows 95. To work with modern operating systems such as Win11 you should use wineevdm. I used 0.9.0 version installer and DMM506.EXE works flawlessly. Software is good for data logging, data plotting and realtime (min 0.5 sec sample-time) monitoring and such. However, in port setup user can only select COM1,2,3 or 4 if your bluetooth module assigned your port different than 1,2,3 or 4 you have to manually change COM port number in device manager to a free port. I wish software has a drop down menu for available COM port list but who am i to disagree :) There are no versions for Linux as i am aware of.

3rd Party Software

Below softwares may be compaitble with PROTEK 506, however i suggest original one cause it have more functions than some of them.

Sigrok: I didn't try but sigrok software may compatible with this multimeter.

TSDmmview: According to this link it should be compatible. I tested it with PROTEK 506 but didn't managed to work. Multimeter connects TX and RX indicators are blinking in below program window but no data seen.

RSMeter: This seems a linux port of RadioShack/Metex 22-168A series multimeters. Not tested.

RadioShack 22-168 Vendor software:(ScopeView Version 1.03) This software is compatible with PROTEK 506. Same version: 2446 signature. Most likely METEX Corp released both 506 and 22-168 software for it but this software seems to have less functions than 506 original software.

PeakTech 4380:(ScopeView Version 1.06) This one works. ScopeView Version 1.06 Same version: 2446 signature. Baud rate can be selected on this version.

ScopeView Version 1.04: SV 1.04In vfd format. Maybe a disk image? I didn't bother to test.

ScopeView Version 1.08: Diffirent version of same program with extended multimeter compability.

Results and Users Guide:

For activating Serial mode on multimeter you should follow these steps:

1. Press the menu key on the meter several times until the "RS-232" blinks.
2. Press the return (enter) key on the meter to set RS232 mode. Now meter is ready to communicate with PC.
3. Run DMM506.EXE
4. Press COM port icon and select COM port 1,2,3 or 4 which is determined on device manager bluetooth serial port.
5. Meter should connected and values should be seen on program according to your meter switch.

I will not go into details how to add bluetooth COM port, just google "add bluetooth com port hc-06" and add your module.

Program GUI:

Plotting and List features:

Cost of Module:

• Components are about 0.588 USD in total but there is minimum order so 3.56 USD if minimum quantities ordered. This price doesn't include shipping.
• HC-06 module price is about 5 USD.
• For PCB manufacturing i paid 3.37 USD for 45 pieces.(I paneled PCB because it's very compact. About 25*25mm.) So about 0.08 USD for one piece. Shipping not included.
• PCB is hand assembled so no price for this item :)

Counterfeit/Clone Bluetooth Modules:

While i was testing HC05 modules i encountered fake modules which cannot be detected by blueflash.

When i desoldered module from its interposer flash chip dropped. As you might see from first picture none of BGA pins are connected to anywhere, all of them connected to module ground. Chip is also fake i believe it doesn't contain any silicon. Module back sides and traces are also different.

Other Mods:

Sleeve Replacement:

My meter was missing terminal sleeve on mA so i designed a replacement. It can be printed with standard FDM printer without supports. Since it's quite small part i suggest %100 infill.