Advantech EKI-1221 - Modbus RTU; Modbus ASCII; Table 2.1: Comparison of Modbus RTU and ASCII Modes; Modbus TCP

EKI-1221/1222/1224 User Manual 8
2.1.2 Modbus RTU
The Modbus/RTU protocol defines how a “master” device polls one or more “slave”
devices to read and write data in real-time over RS-232, RS-422, or RS-485 serial
data communication.
When using RTU mode, each 8-bit byte in a message contains two 4-bit hexadecimal
characters. The main advantage of this mode is that its greater character density
allows better data throughput than ASCII mode for the same baud rate. Neverthe-
less, each message must be transmitted in continuous stream.
2.1.3 Modbus ASCII
When using ASCII mode, each 8-bit byte in a message is sent as two ACSII charac-
ters. The primary advantage of this ASCII mode is that it allows time intervals of up to
one second to occur between characters without causing an error.
2.1.4 Modbus TCP
As a new extension of Modbus/RTU, the Modbus/TCP protocol defines how Modbus/
RTU messages are encoded within and transported over TCP/IP-based networks.
Modbus/TCP is just as simple to implement and flexible to apply as the original Mod-
bus/RTU. The Modbus/TCP protocol is defined by its form of encapsulation for a
Modbus request or response. That means the Modbus request or response data is
encapsulated in TCP frame that has a six-byte header in Modbus/TCP protocol.
Modbus/TCP enables the use of Modbus messaging in an Intranet running the TCP/
IP protocols. Modbus/TCP is most commonly used for Ethernet attachment of PLC’s
or I/O modules to other simple field buses or I/O networks.
Table 2.1: Comparison of Modbus RTU and ASCII Modes
Mode RTU ASCII
Coding System 8-bit binary.
Two hexadecimal
character contained in
each ASCII character of
the message
Hexadecimal.
One hexadecimal
character contained in
each ASCII character of
the message
Bits per Byte 1 start bit,
8 data bits,
1 bit for even/odd parity;
no bit for parity
1 stop bit if parity is used;
2 bits if no parity
1 start bit ,
7 data bits,
1 bit for even/odd parity;
no bit for parity
1 stop bit if parity is used;
2 bits if no parity
Error Check CRC LRC