HEIDENHAIN TNC 335 Technical Manual - Manual (Page 525)

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TNC 360 2 TNC data interfaces 8/95
When a file is transferred, the first block, called the "header", is sent. It consists of the following
characters:
"H" "Name" "M" BCC
(Start of Header): This character indicates the start of the header.
The header contains 'H' - the code for the program (see Section "Saving/reading files"), 'Name' - the
program name and 'M' - the transmission mode(E=input/A=output).
This header ends with character , ending a data transfer block.
The subsequent BCC (Block Check Character) provides additional confidence.
In addition to the parity check of the individual characters (see Section "Checking data"), a parity
check is carried out on a complete transmitted block. The BCC always supplements the individual
bits of the transmitted characters of a data transfer block to give even parity.
Example for formation of BCC:
Character Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
SOH 0 0 0 0 0 0 1
H 1 0 0 1 0 0 0
1 0 1 1 0 0 0 1
5 0 1 1 0 1 0 1
E 1 0 0 0 1 0 1
ETB 0 0 1 0 1 1 1
BCC 0 0 1 1 1 1 1
In this example, the HEIDENHAIN dialog (code 'H') has been used to write program '15' which is
read in via the data interface ('E'). A parity bit is also formed for the BCC (with even parity, the parity
bit of the BCC in this example is given a value '1').
The character is sent after the BCC. This character (XON) is needed for a few devices,
explicitly to demand a reply from them in order to start transfer again.
At the end of each block, a check is conducted to see whether the block has been correctly
transferred. To do this, the receiver forms a BCC from the received block and compares it with the
received BCC. If the received and calculated BCCs are identical, the receiver sends character
(= positive ACKnowledgment), i.e. the data block has been transferred without error.