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