Rockwell Automation 1770-KF2 Data Highway or Highway Plus Interface Module User Manual - Manual (Page 45)

Asynchronous Link Protocols
Chapter 4
4-2
Important: Some Allen-Bradley documentation might refer to
full-duplex and half-duplex protocols as DF1 and polled-mode protocols,
respectively.
Full-duplex protocol conforms closely to ANSI X3.28-1976, combining
features of Subcategories D1 (data transparency) and F1 (two-way
simultaneous transmission with embedded responses).
Full-duplex protocol is used over a point-to-point link that allows
two-way simultaneous transmission. It is relatively difficult to implement
because it requires a system programmer to use interrupts and
multi-tasking techniques. It is intended for high-performance applications
where it is necessary to get the highest possible throughput from the
available communication medium.
Transmission Codes
Full-duplex protocol is character-oriented; it uses the following ASCII
control characters extended to eight bits by adding a zero for Bit 7. See
ANSI X3.4, CCITT V.3, or ISO 646 for the standard definition of these
characters.
Control Character Hexadecimal Code
STX (Start of Text) 02
ETX (End of Text) 03
ENQ (Enquiry) 05
ACK (Acknowledge) 06
DLE (Data Link Escape) 10
NAK (Negative Acknowledge) 15
The term “code” is defined for use in the following paragraphs as an
indivisible sequence of one or more bytes having a specific meaning to
the link protocol. “Indivisible” means that the component characters of a
code must be sent one after another with no other characters between
them. It does not refer to the timing of the characters.
The following codes are used in full-duplex protocol:
Control Codes:
- DLE STX
- DLE ETX BCC/CRC
- DLE ACK
- DLE NAK
- DLE ENQ
Full-Duplex Protocol