PLX30 Series
MBS Protocol
Ethernet and Serial Gateways
User Manual
ProSoft Technology, Inc.
Page 143 of 200
September 19, 2012
Response
An example response to Read Input Status is as shown in Figure C4. The data is
packed one bit for each input. 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 input (1=ON, 0=OFF). The
lower order bit of the first character contains the addressed input, and the
remainder follow. For input 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 a quantity of RTU characters, that is, the
number is the same whether RTU or ASCII is used.
Because the server interface device is serviced at the end of a controller's scan,
data will reflect input status at the end of the scan. Some servers will limit the
quantity of inputs provided each scan; thus, for large coil quantities, multiple PC
transactions must be made using coil status for sequential scans.
Adr
Func
Byte
Count
Data Discrete Input
10197 to 10204
Data Discrete Input
10205 to 10212
Data Discrete Input
10213 to 10218
Error Check Field
11
02
03
AC
DB
35
CRC
The status of inputs 10197 to 10204 is shown as AC (HEX) = 10101 1100
(binary). Reading left to right, this show that inputs 10204, 10202, and 10199 are
all on. The other input data bytes are decoded similar.
Due to the quantity of input statuses requested, the last data field which is shown
as 35 HEX = 0011 0101 (binary) contains the status of only 6 inputs (10213 to
102180) instead of 8 inputs. The two left-most bits are provided as zeros to fill
the 8-bit format.
Read Holding Registers (Function Code 03)
Query
Read Holding Registers (03) allows the user to obtain the binary contents of
holding registers 4xxxx in the addressed server. The registers can store the
numerical values of associated timers and counters which can be driven to
external devices. The addressing allows up to 125 registers to obtained at each
request; however, the specific server device may have restriction that lower this
maximum quantity. The registers are numbered form zero (40001 = zero, 40002
= one, and so on). The broadcast mode is not allowed.
The example below reads registers 40108 through 40110 from server 584
number 11.
Adr Func Data Start Reg Hi Data Start Reg Lo Data #of Regs Hi Data #of Regs Lo Error Check Field
11 03
00
6B
00
03
CRC
