Section 4.7 How Inview Sign Communication Protocol Is Used With Modbus Ascii Protocol - Allied Motion Spectrum Controls 2706-P22-SC User Manual

Section 4.7 How InView Sign Communication Protocol is used with Modbus ASCII
Protocol
Beg of
Address
Frame
2-char
:
16 bits
Beg of
Address Function
Frame
2-char
:
16 bits
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 read a 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.
ASCII Message Frame Format
Function
2-char N × 4-char
16 bits N × 16 Bits
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.
The following table shows the format of the 01 Frame used for the downloading
of messages to the displays.
Function 01 Used for Downloading of Messages
Starting
Register
Lo
2-char 2-char
16 bits 16 bits
Messages are downloaded to all displays on the Modbus ASCII display network.
A broadcast address is used that requires no response from the displays. Modbus
ASCII protocol uses address 00h for broadcasting information from one device to
the next. This is generally reserved for PLC-to- PLC communications. For this
reason, address 255 (0×ff) is used.
Mode of Transmission
Mode of transmission is the format in which messages are transmitted over the
network. Characteristics of the Modbus ASCII system are:
Coding Systems – hexadecimal (uses ASCII printable characters: 0-9,
•
A-F).
Modbus allows the format for data transmission.
• Communication rate: user selectable
• Format: Number of bits per character – 1 start bit, 7 data, 1 (optional)
parity bit, 1 or 2 stop bits.
Chapter 4: InView Protocol
Error
Data
Check
2-char
Starting
Register Data
Hi
N × 4-
2-char
char
16 bits N × 16
Bits
Ready to receive
EOF
response
CR LF
Ready to
Error
EOF
Check
response
2-char
CR
16 bits
User's Manual 0300277-03 Rev. A
4-9
receive
LF