Rockwell Automation 1770, D17706.5.16 Ref Mnl DF1 Protocol Command - Manual (Page 31)

2–2 Understanding DF1 Protocol
Publication 1770Ć6.5.16 - October 1996
A link protocol is a set of programming rules for interpreting the
signals transmitted over a physical link. A protocol, such as DF1:
•
carries a message, error free, from one end of the link to the other
It has no concern for the content of the message, the function of
the message, or the ultimate purpose of the message.
For example, with full-duplex DF1 protocol, it accomplishes this
by attaching a check character (BCC) or check characters (CRC)
to the end of each command and reply. The device receiving the
command or reply then verifies the BCC or CRC and returns an
ACK—if the BCC or CRC is acceptable—or an NAK—if the
BCC or CRC does not check.
•
indicates failure with an error code
Internally, the link protocol must delimit messages, detect and
signal errors, retry after errors, and control message flow.
DF1 protocol is an Allen-Bradley data-link layer protocol that
combines features of subcategories D1 (data transparency) and F1
(two-way simultaneous transmission with embedded responses) of
ANSI x3.28 specification. There are two categories of DF1
protocol:
•
half-duplex protocol (master-slave communication)
•
full-duplex protocol (peer-to-peer communication)
HalfĆduplex Protocol
Half-duplex protocol is a multidrop protocol for one master and one
or more slaves. With half-duplex protocol, you can have 2 to 255
nodes simultaneously connected on a single link; this link operates
with all nodes interfaced through half-duplex modems. (For a list of
devices that can be used as masters and slaves, see page
3–2 . For
more on half-duplex protocol, refer to Chapter 3, “Using Half-duplex
Protocol to Send and Receive Messages.”)
To implement half-duplex protocol, use the following
communication characteristics:
•
8 bits per character
•
no parity
•
1 stop bit
DF1 Protocol