Rockwell Automation 2706-P42_P43_P44_P72_P74_P92_P94 InView Marquee Message Display User Manual - Manual (Page 77)

Publication 2706-UM016D-EN-P - March 2006
InView Protocol 4-11
Below are several terms used throughout the Modbus ASCII section.
• ASCII - American Standard Code for Information Interchange.
• LRC - Longitudinal Redundancy Check used for error checking.
• RTU - Remote Terminal Unit.
• Query – Sending information to a node address where a
response is expected.
• Response – A response is given to a Query.
• Transmission – Sending information to all node addresses where
no response is expected.
How InView Sign
Communication Protocol is
used with Modbus ASCII
Protocol There are two different modes of transmission used with Modbus
protocol, Modbus ASCII and Modbus RTU. In this case, Modbus ASCII
is used. Modbus ASCII protocol is used to accomplish such functions
as Priority Messaging, Add a Message, Remove a Message, Update
Variables, and reading register in the Message Queue or Variable.
The basic framing format that is used in the Modbus ASCII is shown in
the section Add a Message on all Displays on page 4-7.
ASCII Message Frame Format
Downloading messages to the displays using Modbus ASCII protocol
requires a special function in order to pass Allen-Bradley’s InView
industrial protocol to the displays. To accomplish this, a Modbus 01
function or frame is specifically used to encapsulate the InView
protocol. The downloading of messages that are created using
automation software automatically encapsulates the protocol in the
Modbus ASCII string.
To encapsulate the InView protocol a Modbus ASCII header and
trailer are used. The automation software has message length range
from 50 to 450 Bytes. This is only the maximum length of the InView
protocol that is encapsulated within the Modbus ASCII protocol.
When downloading messages on a Modbus network, the maximum
length of the Modbus ASCII and the InView protocol is 500 bytes.
Beg of
Frame Address Function Data Error
Check EOF Ready to
rec. resp
: 2-char
16 bits 2-char
16 bits N x 4-Char
N x 16-Bits 2-char
16 bits CR LF