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1 Introduction
11
return from the sub-program to the calling program.
DNC Channel
A program contained in an external unit (e.g., in a computer) can also be executed without storing it
in the control's memory. Now the control will read the program, instead of the memory, from the
external data medium through the RS232C interface. That link is referred to as "DNC channel". This
method is particularly useful for the execution of programs too large to be contained in the control's
memory.
The DNC channel is a protocol-controlled data transfer channel as shown below.
The above mnemonics have the following meanings (and their ASCII codes):
BEL (7): The control requests the sender to establish the communication. The control issues
L again unless ACK is returned in a definite length of time.
ACK (6): acknowledgment.
NAK (21): Spurious data transfer (e.g. hardware trouble in the line or BCC error). The
transfer of BLOCK has to be repeated.
DC1 (17): Transfer of the next BLOCK has to be started.
DC3 (19): Interruption of communication.
BLOCK:
– Basically an NC block (including the terminating character s) and the checksum
thereof (BCC) stored in 7 bits as the last byte of the block (bit 7, the
uppermost one, of BCC is invariably 0). No SPACE (32) or some other
character of lower ASCII code may be contained in the block.
– EOF (26) (End Of File), a signal is transferred by the Equipment ("sender") to
interrupt the communication.
For the DNC mode, set the second physical channel (only that one is applicable as a DNC channel)
for 8-bit even-parity mode.
A main program executed from the DNC channel may have a linear sequence only. This does not
apply to subprogram or macro (if any have been called) however, they must be contained in the
memory of control. In the event of a departure from the linear sequence in the main program
(GOTO, DO WHILE), the control will return error message 3058 NOT IN DNC. If the control
detects a BLOCK error and returns NAK, the BLOCK has to be repeated.
Controller:
Equipment:
<
BEL > DC1 NAK/ACK DC3
ACK > BLOCK
<