Data Transfer With Block Check Character (Bcc) - HEIDENHAIN TNC 415 Technical Manual

3.2 Data Transfer with Block Check Character (BCC)

3.2.1 General
This protocol is specific to HEIDENHAIN and operates with different control characters and an
additional data check feature when transferring.
The protocol is set with the following operating modes:
- FE1 mode
- FE2 mode
- EXT1/EXT2/EXT3 mode (selectable)
The data transfer protocol is fully identical for all these modes except for the FE1 mode in which a
command sequence is automatically output at the beginning to request the contents directory from
the peripheral unit.
In the freely configurable modes (EXT1/EXT2/EXT3) the following control characters (<SOH>,
<ETB>, <STX>, <ACK>, <NAK>, <ETX>, <EOT>) can be defined at will as ASCII characters
(see section "Freely configurable interfaces").
When a file is transferred the first block - the so-called Header - is transmitted, consisting of the
following characters:
<SOH>"H" "Name" "M" <ETB>BCC<DC1>
<SOH> (Start of Header): This character identifies the beginning of the header.
The header contains the identifier "H" of the program (see section "Saving/reading files"), the program
name "Name" and the transfer mode "M" (E=Input/A=Output).
The header terminates with the character <ETB> which ends a data transfer block.
The next character BCC provides an additional data check:
As well as checking the parity of the individual characters (see chapter "Checking data") the parity of
the complete transferred block is also checked. The BCC (Block Check Character) always rounds the
individual bits of the transferred characters in a data transfer block to even parity.
Example of BCC generation:
Character Bit 6
SOH 0
H 1
1 0
5 0
E 1
ETB 0
BCC 0
8-36 TNC 407/TNC 415/TNC 425
Bit 5 Bit 4 Bit 3
0 0 0
0 0 1
1 1 0
1 1 0
0 0 0
0 1 0
0 1 1
2 TNC data interfaces
Bit 2 Bit 1 Bit 0
0 0 1
0 0 0
0 0 1
1 0 1
1 0 1
1 1 1
1 1 1
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