300-R010-45G Datasheet, PDF (27/36 Pages) Magnetrol International, Inc. – Echotel® Model 300/301 Ultrasonic Non-Contact Transmitter For Level/Volume/Open Channel Flow

MODBUS PROTOCOL
IMPLEMENTATION
This protocol guide explains the operation of the Modbus
protocol per Modicon document PI-MBUS-300 Rev J.
The Magnetrol implementation of the Modbus protocol pro-
vides a standard form of digital communications. An effort
has been made to parallel current implementations to the
greatest extent possible, so that Magnetrol products com-
municate with existing Modbus masters.
Check compatibility carefully to ensure that the Model
300/301 is configured for the data format expected by the
host computer. Exceptions made because of the unique
requirements of the Magnetrol installation have been noted.
This is no guarantee, however, that the interpretation made
here will be the same as that followed by the Modbus master.
The Magnetrol implementation of the Modbus protocol pro-
vides for the passing of measured and calculated variables,
configuration information, and diagnostics in data registers.
Data is sent in these registers as floating point values, integer
values, numeric codes related to configuration lists, status
summary words (packed bits), or individual flags (single bits).
One master and up to 31 units may be multi-dropped on a
single RS-485 communication bus.
The Modbus functions implemented in Model 300/301 are
listed in Table 3.
CONFIGURATION
The Modbus port on the Model 300/301 must be configured
to establish communications. The keypad allows the user to
set the Magnetrol Modbus port to match the Modbus mas-
ter. See page 24 for configuration instructions.
Magnetrol addresses provide unique identification for the
host. This address may range from 1 to 247 and must be
unique. Each Model 300/301 only responds when a query
has been sent to its unique address by the host.
Magnetrol units support the Remote Terminal Unit (RTU)
mode of transmission. RTU provides improved error detec-
tion capabilities and higher throughput than the ASCII mode.
The RS-485 port must be configured for a transmission
speed (baud rate). Allowable values are 1200, 2400, 4800, or
9600 bits per second.
Table 3 Modbus Functions
Function
Code
Function
Information Type
MODBUS
Nomenclature
01
Read
02
Read
03
Read
04
Read
05
Write
06
Write
15
Write
16
Write
1-bit coils
1-bit coils
16-bit register/integer
16-bit register/integer
16-bit register/coils
16-bit register
16-bit register/coils
16-bit register
Read coil status
Read input status
Read holding registers
Read input registers
Force single coil
Preset single register
Force multiple coils
Preset multiple registers
Exception:
Because the Model 300/301 does not distinguish between inputs and outputs, function codes 01 and 02 (as they apply to bits) and
function codes 03 and 04 (as they apply to numeric values) refer to the same data registers.
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