Multimeter ICs

Protocol

The chip periodically sends 14-byte packets at 2400 baud, 8n1. The upper nibble of each byte indicates the byte number. The payload is composed of the lower nibbles, and is a 1-1 mapping of the LCD segments. The downside to this protocol structure is that transmission errors in the LSB nibbles cannot be detected. There is no checksum or CRC in the packet.

Packet structure

Byte	Bits 7-4	Bit 3	Bit 2	Bit 1	Bit 0
0	0x1	AC	DC	Auto	RS232
1	0x2	Negative	1A	1B	1C
2	0x3	1D	1E	1F	1G
3	0x4	DP1	2A	2B	2C
4	0x5	2D	2E	2F	2G
5	0x6	DP2	3A	3B	3C
6	0x7	3D	3E	3F	3G
7	0x8	DP3	4A	4B	4C
8	0x9	4D	4E	4F	4G
9	0xa	u	n	k	Diode
10	0xb	m	%	M	Beep
11	0xc	Farads	Ohms	Rel	Hold
12	0xd	A	V	Hz	Low battery
13	0xe	User bit 3	User bit 2	User bit 1	User bit 0

Segment lettering

	C
B		G
	F
A		E
	D

Photos

<gallery> File:Voltcraft vc820 fs9721 lp3.jpg|FS9721_LP3 on Voltcraft VC-820 File:Tp4000zc_chip.jpg|FS9721 (?) on TekPower TP4000ZC <gallery>