7526 DCT Reference

Control Characters

Certain ASCII characters are used in the network protocol as control characters. Because binary files (files which can contain all 256 8-bit characters) can be transferred to the 7526 DCT over the network, the protocol defines a use of Escape characters to avoid having binary data misinterpreted as network control characters. To make transmission of the following control characters transparent to the protocol, each character is prefixed with an ESC (1Bh) character as it is transmitted. Additionally, the STX character is suffixed with an ESC character and transparency is achieved by inserting an additional ESC character.

 
  Character       Transmit As
  _________       ___________
 
  SOH (01H)       ESC SOH
  STX (02H)       ESC STX ESC
  ETX (03H)       ESC ETX
  NAK (15H)       ESC NAK
  ESC (1BH)       ESC ESC

Computers which transmit data to a 7526 DCT must insert these transparency characters in the data stream, and be prepared to handle transparency characters returning from the DCT.

The ESC character insertion must not force a transmission record to exceed the 255-byte limit for RS-422/485 serial connection or the 472-byte limit for Ethernet connection. When transmitting binary data, count the number of ESC characters required and reduce the data count in each record accordingly.

Note:

The transmission record limit from the terminal is 255 bytes and is configurable between 128 and 255 bytes (refer to Table 5. , offsets 42-45). In the 7525 compatibility mode, transaction record size is 128 bytes. The default for the 7526 DCT is 128 bytes.