Telepen

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Telepen is a name of the barcode symbology designed to encode all 128 ASCII characters without using shift characters for code switching, and only using two different widths for both bars and spaces.^[1] (Unlike Code 128, which uses shifts and four different element widths^[2]). The symbology was devised by George Sims of SB Electronic Systems Ltd.^[3][4] Telepen was originally designed in the UK in 1972.

Unlike most linear barcodes, Telepen does not define independent encodings for each character, but instead operates on a stream of bits. It is able to represent any bit stream containing an even number of 0 bits, and is applied to ASCII bytes with even parity, which satisfies that rule. Bytes are encoded in little-endian bit order.

The string of bits is divided into single 1 bits and blocks of the form 01^*0. That is, blocks beginning and ending with a 0 bit, with any number of 1 bits in between.

These are then encoded as follows:

• "1" is encoded as a narrow bar-narrow space
• "00" is encoded as a wide bar-narrow space
• "010" is encoded as wide bar-wide space
• Otherwise, the leading "01" and trailing "10" are both encoded as narrow bar-wide space, with additional 1 bit in between coded as described above.

Wide elements are 3 times the width of narrow elements, so every bit occupies 2 narrow elements of space.

Barcodes always start with ASCII _ (underscore). This has a code of 0x5F, so the (lsbit-first) bit stream is 11111010. Thus, it is represented as 5 narrow bar/narrow space pairs, followed by a wide bar/wide space.

Barcodes always end with ASCII z. This has (including parity) a code of 0xFA, so the (lsbit-first) bit stream is 01011111. This is encoded as a wide bar/wide space, followed by 5 narrow bar/narrow space pairs. Each end of the bar code consists of repeated narrow elements terminated by a pair of wide elements, but the start has a wide bar first, while the end terminates with a wide space.

In addition to per-character parity bits, a Telepen symbol includes an overall modulo-127 checksum.