V&A VA18B

Mastech VA18B

The Mastech VA18B a.k.a. V&A VA18B (V&A is a brand of Mastech) is a 6000 counts, CAT II (1000V) handheld digital multimeter with USB connectivity.

See Mastech VA18B/Info for more details (such as lsusb -vvv output) about the device.

Model overview

It seems there have been at least two different revisions of the multimeter (or at least of the PC-Link cable). The older one used a Sunplus SPCP825 USB-to-serial chip (see here, here, or also check the drivers from here), the newer revisions use a Prolific PL2303HX chip. Both should work fine with sigrok, as long as the respective driver creates a "virtual COM port" / serial port device sigrok can connect to.

Device	Rebranded?	PC interface chip	Vendor software	Comments
Mastech VA18B (old)	no	Sunplus SPCP825	"PC-LINK"	Manual: VA18B.pdf
Mastech VA18B (new)	no	Prolific PL2303HX	"PC-LINK"	Manual: VA18B.pdf. PCB silk screen: "VA18B 20070302".
Amarad Hellas Electronic VA18B	yes (VA18B)	?	?	See also this photo. PCB silk screen: "VA18B 20070302".
Velleman DVM1200	yes (VA18B)	?	"PC-LINK"	—
Maxwell MX-25328	yes (VA18B)	?	"Maxwell Universal Program"	Maxwell wrote their own PC software, apparently (works fine with the VA18B if you select "25328" as device in the software).
Peaktech P3375	likely (VA18B)	?	?	Very likely a rebranded Mastech VA18B (1, 2, 3).

Hardware

Multimeter:

• Main chip: Unknown, it's very likely a bare die under the black blob (see photos). The pads suggest it's a 100pin device.
• Display: ?
• Precision, very low power, CMOS dual opamp:: 27L2C U85T CN58
• ?: 7530-1 1129
• Crystal: 4MHz
• Fuses: 750mA/600V (5x20mm), 10A/600V (6x30mm)

USB cable:

• USB to serial chip: Prolific PL2303HX YR0903A
• Crystal: 12MHz

Photos

Multimeter:

• 

  Package, front

• 

  Package, back

• 

  Package, contents

• 

  Device, front

• 

  Device, back

• 

  Device, open 1

• 

  Device, open 2

• 

  PCB, front

• 

  PCB, front, 1

• 

  PCB, front, 2

• 

  PCB, front, 3

• 

  PCB, back

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  27L2C

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  7350-1

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  Crystal

• 

  LCD

USB cable:

• 

  USB cable

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  USB cable PCB, front

• 

  USB cable PCB, back

Protocol

14-byte LCD segments over USB-2-serial (Prolific PL2303HX chip, 2400 baud, 8n1), see various links below.

The transmission of the 14-byte chunks happens roughly every 300ms (measured using a logic analyzer at 24MHz samplerate, on the receiver IR diode on the USB cable and the RX pin of the PL2303HX). Sometimes it's 300.15ms, then 325.16ms, then 300.15ms, then 325.16ms again, and so on. This timing seems to be quite consistent.

However, only exactly 8 of these 14-byte chunks are spaced at 300ms. Every 9th chunk of 14 bytes will then be 600ms apart, for (yet) unknown reasons. This is not due to the IR transmission, the same effect can be verified when measuring the respective timing on the multimeter itself (before the data gets sent via IR).

To enable output to the PC on the multimeter you have to keep the Hz/DUTY key pressed while powering on the device. However, it will auto-poweroff after roughly 1 hour, even in this mode. To avoid that, press both the Hz/DUTY and the SELECT key during power-up (see manual, page 9, section 2.3.1).

Resources

• Protocol descriptions: [1], [2], [3]
• EEVblog forums: Product review: DMM latest V&A Mastech VA18B
• Other teardowns: 1, 2, 3 (VA18B, but the PCB silkscreen says "VA17B 20060413")