Checksums are used to detect errors in the transmission of data. The idea is that the whole unit of data can be identified by a single number, a checksum. If the checksum does not match its expected value, errors have occurred.

Many types of barcodes employ checksums. In several other types, checksums are optional. Those that incorporate checksums are more robust and reliable.

Checksum Example

The simplest checksum is to add up the value of the individual bytes in the data. Given the data of "CAT", the ASCII values of the letters are as follows:

• C - 67
• A - 65
• T - 84

Summing these values gives 216, and the whole package might be given the notation "CAT(216)". So if we received "CAS(216)" we’d do the same steps:

• C - 67
• A - 64
• S - 83

We would sum these values and receive 215. Since 215 does not equal 216, we know that an error has occurred.

This simple sum of the characters can be fooled by switching the order but not the value of the data, e.g. "TAC" and "CAT" have the same checksum. So more sophisticated checksum methods have been developed which reduce (but never totally eliminate) the chance of incorrect data giving the correct checksum.

Error Correction

In the example above, we can detect that "CAS(216)" has a data transmission error, but we can't correct it becausee we don't know which character caused the problem. "BAS(216)" or "CAT(216)" could be the correct data.

To be able to correct data transmission errors, Reed-Solomon Error Correction is performed. More often, advanced barcodes use this system. There is a trade-off here however, because as the barcode is made more robust against transmission errors, the barcode has to store more ECC data in its data.

Platform-Specific Notes

Checksums by Barcode Type

Barcodes with No Checksum	Barcodes Requiring a Checksum	Barcodes with Optional Checksum	Barcodes with Error Correction
Add 2	Code 128	Code 39	Australia Post 4 State
Add 5	Code 93	Code 39 Extended	Aztec
Codabar	Code 93 Extended	Industry 2 of 5	Data Matrix
Code 32	IATA 2 of 5	Interleaved 2 of 5	MicroPDF417
DataLogic 2 of 5	EAN-128	Inverted 2 of 5	PDF417
Patch Code	EAN-13	BCD Matrix	QR Code
	EAN-8		UPU S18 4-State
	GS1 DataBar
	Intelligent Mail
	Matrix 2 of 5
	Planet
	PostNet
	Royal Post 4-State
	UPC-A
	UPC-E

---

For more information, see the Barcode Xpress SDK product page or try our online demos.