Specifications

56
Schneider Electric
The Modbus Protocol
Difference between ASCII and RTU frames
Some devices can use either of the two formats:
b With the RTU format the user can obtain higher data throughput with the same
transmission speed.
b The ASCII format provides greater flexibility on inter-byte timing (up to 1 second)
and provides truly asynchronous transmission.
Differences between Modbus and Modbus+
protocols
b In the Modbus protocol, the receivers read the address and thus identify the
module to which the message is addressed. The receivers also determine the length
of the message and can thus detect truncated messages.
b In Modbus+ the transmitter converts the Modbus address to an address with the
node and access path on the network.
Modbus / Jbus
Definition : ASCII, American Standard
Code for Information Interchange.
Definition : RTU, Remote Terminal Unit.
Both of these frame formats can be used in
the Modbus protocol, but they are
incompatible with each other. About 95%
of modules that communicate over Modbus
use RTU frames.
Definition : The Modbus protocol uses
frames with standard beginnings and ends.
The address is placed at the beginning of
the frame.
Definition : The Modbus+ protocol uses
frames with start and end delimiters that are
specific to the network. The destination
address is integrated in the protocol.
The Jbus protocol uses part of the Modbus
protocol.
Note: Products (modules) may contain only
part of the Modbus functions (see product
documentation).
Frame structure Jbus Modbus
Slave no.
Function code
Frame length
CRC
Frame detection*
1 to 255
1 to 16 (except 9 & 10)
Maximum 255 bytes
CRC16
silence > 3 characters
1 to 247
1 to 24
Maximum 261 bytes
CRC16
silence > 1.5 or > 3.5
characters
* This difference is unimportant at throughputs greater than 1200 bits/s because the times are
shorter than device processing time (turnaround time).
Standard functions
F1 Read n bits at @ 0 to FFFF
(1 < n < 2000)
Read n bits at @ 1 to 9999
F2 Read n bits at @ 0 to FFFF
(1 < n < 2000)
Read n bits at @ 10001 to
19999
F3 Read n words at @ 0 to FFFF
(1 < n < 125)
Read n words at @ 40001 to
49999
F4 Read n words at @ 0 to FFFF
(1 < n < 125)
Read n words at @ 30001 to
39999
F5 Write 1 bit at @ 0 to FFFF Write 1 bit at @ 1 to 9999
F6 Write 1 word at @ 0 to FFFF Write 1 word at @ 40001 to
4999
F7 Fast read 8 user bits Read exception status (8 bits).
Signal errors in the equipment
F15 Write n bits at @ 0 to FFFF Write n bits at @ 1 to 9999
F16 Write n words at @ 0 to FFFF Write n words at @ 40001 to
49999
Note: @ Modbus begin at "1", while @
JBus begin at "0".
Guide to Integration
Modbus / Jbus