TPU2000/2000R Modbus/Modbus Plus/ Modbus TCP/IP Automation Guide
46
Function 01 - Read Coil Status
Modbus Host
E
C
Modbus Slave Addr =1
Read from
0X Mapping
Slave
Addr.
Funct.
Code
01
Start
Addr
HI
Start
Addr
LO
Coils
Read
HI
Coils
Read
LO
Error
Check
EOM
SOM
Slave
Addr.
Funct.
Code
01
Byte
Count
*
Data
Byte
1
…..
Data
Byte
NNN
Error
Check
EOM
SOM
Byte 1 …2……..3…….4…….5……6……..7….
MSB LSB
87
65
4321
SOM = Start of Message
EOM = End of Message
Figure 5-14. Modbus Protocol Function 01 Frame Format
Function Code 1 (Read Coil Status) – Read Only Data
The 0X read command allows for access of Logical and Physical Input data. The information listed in Tables 1,2,
and 3 is that which is reported in real time. In other words, if the bits are polled as per the table, the status of
each data bit is reported at the time the data is requested. If the data is momentary in nature, then access of
status is dependent upon reading the information at the time the function or signal is present.
Table 5-1 lists the Logical Output Single Bit Data. The data listed within the table includes real time status bits
which may be briefly reported status bits. In other words, they follow the real time status of the point. Other
points reported in the table are latched.
A Latched point (sometimes referred to as Sealed In Output Point). These points stay energized until they are
reset by a group control function. The function is reset via the method described in the 4X control explanation
and an example is shown in Figure 5-15.
Momentary data reporting is available at the present time. Some bit statuses are brief in reporting nature.
Modbus and Modbus Plus do not have a method of timestamping events, nor is there a “protocol defined” method
to ensure that an event is not lost. ABB incorporates a method called “Momentary Bit Status Reporting” allowing
a host to poll a protective device at any time and ensure that a contact change notification occurs. The method
shall be explained later in this document.
If data is requested from memory addresses not defined within this document, a Modbus Exception Code shall be
generated.
Figures 5-15 and 5-16 illustrate a simple example of a host requesting data from a TPU2000R relay where
Physical Relay Coil Status is requested of the TPU2000/TPU2000R. The example illustrates that data is
requested in the Modbus ASCII frame format illustrated in Figure 5-15, raw data received by the host is decoded
from ASCII to HEX. The host as illustrated in Figure 5-16 parses the individual status bits.
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