HEIDENHAIN TNC 407 (243 020) Technical Manual User Manual
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In this example program "15" that has been written in HEIDENHAIN dialog ("H") is downloaded
across the data interface (E"). A parity bit is also generated for the BCC (with even parity the BCC
parity bit in this example is assigned significance "1").
The character <DC1> is transmitted after the BCC. This character (XON) is necessary for certain
devices in order to explicitly request an answer from them and to reactivate the transfer.
The transfer of the <DC1> character can be suppressed in modes EXT1, EXT2 and EXT3 by setting
bit 3 in machine parameter 5020.X ("Transmission stop by DC3") to zero (see section "Freely
configurable interfaces").
The <DC1> character is not required when reading in a file in the format with BCC.
At the end of every block the receiver checks that it has transferred correctly. To do this the receiver
computes a BCC from the received block and compares it with the received BCC. If the received
BCC and the computed BCC are identical, the receiver transmits the <ACK> character (= positive
checkback), i.e. the data block has been transmitted without error.
If the two BCCs are not identical then the receiver transmits the <NAK> character (= negative
checkback), i.e. the data block was not transmitted correctly and must be re-transmitted. This
process is repeated three times, then the error message ("TRANSFERRED DATA INCORRECT E {XE
"TRANSFERRED DATA INCORRECT E") is output and the transfer is aborted.
If the header is acknowledged with <ACK> however the first file block can be transmitted:
<STX>0 BEGIN PGM 1 MM <ETB> BCC <DC1>
The beginning of a data block is always identified by the control character <STX>. The other control
characters in this block are identical with the control characters in the header.
If the block is acknowledged by <ACK> then the next program block is transmitted, with <NAK> the
same block has to be re-transmitted etc.
Once the last program block has transferred successfully (acknowledged by <AC>) the transmission
is terminated by the characters <ETX> (end of text) and <EOT> (end of transmission).