300-R010-45G Datasheet, PDF (28/36 Pages) Magnetrol International, Inc. – Echotel® Model 300/301 Ultrasonic Non-Contact Transmitter For Level/Volume/Open Channel Flow

FUNCTIONS AND DATA FORMAT
The Modbus data in the Model 300/301 is arranged in inte-
ger register, floating point registers, and status bits. The
assignments for these registers are found in the tables on
pages 30 and 31. Function codes 03, 04, 06, and 16 are used
with integer registers.
Function codes 01, 02, 05, and 15 are used with status bits.
Both integer and floating-point registers have space
reserved to reorder the registers for maximum communica-
tion throughput.
A complete description of all the preceding commands,
except floating point, can be found in Modicon Modbus
Protocol Reference Guide, document number PI-MBUS-300
Rev J.
INTEGER REGISTERS
Integer registers are the most commonly used type of
Modbus data and are supported by most Modbus hosts. In
the Magnetrol implementation, the Modbus registers are
arranged in one of the following four formats:
1. Integer Data: A scaled number from 0 to the maximum
Modbus integer.
2. Character Data: Two ASCII characters per 16-bit regis-
ter (i.e., date, password).
3. Coded Data: Multiple choice configuration data chosen
from a coded list.
4. Packed Bits: Register form of 16 packed single bits
The integer, character, and coded data registers contain all of
the information needed to configure and read process data.
Any integer register may be read with function code 03 or
function code 04. These same registers may be written one
at a time with function code 06 or multiple registers can be
written with function code 16.
PROGRAMMING
The serial communication for the Model 300/301 is designed
to communicate with a host computer over two wires in a
multidrop system.
Each transmitter in the system is assigned a unique address
that the computer uses to communicate with that transmitter.
COMMAND/RESPONSE SEQUENCE
The system uses half duplex communication with the host
computer acting as the master and the 300/301 transmitters
acting as the slaves. The 300/301 transmitter is designed for
1 master computer and up to 32* slaves. Each slave has a
unique address between 1 and 247 associated with it. The
address is used to wake up the transmitter and interrogate it
for data.
The host will send down a command to the unit using the
address to wake it up. The unit will then analyze the com-
mand and retrieve the appropriate data. After sending back
a response, the unit will return to the sleep mode.
Only the host computer can issue a command. Each com-
mand contains the unit address, the function code, the data,
and the error checking valve. Magnetrol products support
the CRC error checking valve.
*See cable requirements for the maximum number of units allowed
for intrinsically safe operation.
RXD/TXD +
RXD/TXD –
Figure 17
Command to Transmitter
Command from Host
• Transmitter Address
• Function Code
• Data Bytes
• Error Check
Response from Transmitter
• Transmitter Address
• Function Code
• Data Bytes
• Error Check
Response to Host
Figure 18
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