ProSoft Technology PLX31-EIP-SIE - Manual (Page 139)

PLX3x Series MBS Protocol
Ethernet and Serial Gateways User Manual
ProSoft Technology, Inc. Page 139 of 218
September 17, 2014
7.4 MBS Reference
7.4.1 Modbus Protocol Specification
Read Coil Status (Function Code 01)
Query
This function allows the user to obtain the ON/OFF status of logic coils used to
control discrete outputs from the addressed server only. Broadcast mode is not
supported with this function code. In addition to the server address and function
fields, the message requires that the information field contain the initial coil
address to be read (Starting Address) and the number of locations that will be
interrogated to obtain status data.
The addressing allows up to 2000 coils to be obtained at each request; however,
the specific server device may have restrictions that lower the maximum quantity.
The coils are numbered from zero; (coil number 1 = zero, coil number 2 = one,
coil number 3 = two, and so on).
The following table is a sample read output status request to read coils 0020 to
0056 from server device number 11.
Adr Func Data Start Pt Hi Data Start Pt Lo Data # Of Pts Ho Data # Of Pts Lo Error Check Field
11 01 00 13 00 25 CRC
Response
An example response to Read Coil Status is as shown in Figure C2. The data is
packed one bit for each coil. The response includes the server address, function
code, quantity of data characters, the data characters, and error checking. Data
will be packed with one bit for each coil (1 = ON, 0 = OFF). The low order bit of
the first character contains the addressed coil, and the remainder follows. For coil
quantities that are not even multiples of eight, the last characters will be filled in
with zeros at high order end. The quantity of data characters is always specified
as quantity of RTU characters, that is, the number is the same whether RTU or
ASCII is used.