Modbus Rtu Emulation; Figure 5-5. Modbus Rtu Format; Figure 5-6. Rtu Frame Format - ABB TPU2000 Technical Manual

TPU2000/2000R Modbus/Modbus Plus/ Modbus TCP/IP Automation Guide
ABB TPU2000 and TPU2000R, does not allow for this override, however several programmable logic controller
manufacturers allow for this.
The receiving device determines that a frame is on the network by sensing the first character (: colon) and then
determining that the message address is the same as that assigned to itself. If the Modbus device does not
receive a carriage return line feed (lf cf 0A 0D) within an appreciable amount of time, the host will timeout. The
length of characters in the message determines Timeout. Modbus ASCII will timeout is the time delay between
each character exceeds 1 second delay between each character's transmission. If 100 characters are required to
transmit a complete Modbus ASCII frame, then the timeout for the message could be in excess of 100 seconds
for that specific exchange.

Modbus RTU Emulation

In contrast to the ASCII representation, Modbus allows for no encoding of the transmitted or received data
message. If a data byte of 00 (zero zero) is sent to an IED from a Host, the data would be sent as a single byte of
data (binary 0000 0000). If data would be sent as an ASCII data string the data would be composed of the
encoded ASCII string 30 30 hex (binary 0011 0000 0011 0000). The Modbus RTU emulation is twice as efficient
as Modbus ASCII mode.
4 Char
Delays
Least Significant Bit ................................................Most Significant Bit
START
START
Figures 5-5 and 5-6 illustrate the format of the Modbus RTU emulation. An analysis of each frame is illustrated in
Figure 5-6. When selecting a common frame size, (as explained in the configuration setup examples), parity,
word length, and stop bits are selected to form a 11 bit data frame (1 start bit + 8 data bits + 1 stop bit + 1 Parity
bit "OR" 1 start bit + 2 stop bits + 8 data bits + NO Parity = 11 bits per frame). It is important to note this
distinction since if TPU2000, and TPU2000R, device attachment is to occur through a device, the device must
support 11 bit asynchronous data framing.
Modbus ASCII protocol synchronizes host to IED messaging through monitoring the leading character (: colon).
Modbus RTU synchronizes the host to IED messaging through time delays.
RTU timeout depends on the following rules.
START ADDRESS
8 Bits

Figure 5-5. Modbus RTU Format

RTU Framing
1 2 3 4
With Parity Checking
1 2 3 4
Without Parity Checking

Figure 5-6. RTU Frame Format

FUNCTION
DATA
8 Bits N * 8 bits
5 6 7 8 PARITY
5 6 7 8 STOP
LRC END
16 Bits 4 Char
Delays
STOP
STOP
Modbus RTU emulation. Modbus
40