300-R010-45G datasheet(28/36 Pages) MAGNETROL

28 / 36 page

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.

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.

FUNCTIONS AND DATA FORMAT

PROGRAMMING

RXD/TXD +

RXD/TXD –

Figure 17

Command from Host

Response from Transmitter

• Transmitter Address

• Function Code

• Data Bytes

• Error Check

• Transmitter Address

• Function Code

• Data Bytes

• Error Check

Response to Host

Command to Transmitter

Figure 18

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