Modbus Rtu; Modbus Ascii; Figure 2.1 Modbus System Architecture 1 - Advantech EKI-1222 User Manual

The basic system architecture is as below:
P1 P2 Status
Default
1
2
Rx Tx Rx
Tx
1 3
Rx Tx
Tx Rx
2
4
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.
Mode
Coding System
Bits per Byte
Error Check
PLC
Modbus / TCP
Master Device
Host (PC)
P1 P2
Default Status
1
EKI 1222
2
Rx
Tx
1
Rx Tx
2 P1 P2
Status
Default
1
EKI 1221
2
Rx
Tx
1

Figure 2.1 Modbus System Architecture 1

Figure 2.2 Modbus System Architecture 2
RTU
8-bit binary.
Two hexadecimal character co
tained in each ASCII character of
the message
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
CRC
5
PC
ASCII
Hexadecimal.
One hexadecimal character co
tained in each ASCII character of
the message
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
LRC
EKI-122X Series User Manual