Dwyer MS2-M User Manual

Page 4

Modbus

Communication Protocol functions

The device supports the following functions

The String data type is read as a stream of ASCII characters, with the first character
sent in the MSB of the first register, and the second character sent in the LSB of
the first register and so on. If the string is shorter than the allotted size, the
remaining bytes will be zero padded.

Holding Registers

The holding registers allow configuration of the Velocity K value and Area
associated with velocity and flow calculations. By default, both the Velocity K value
and Area are set to 1.0. In order to read a correct velocity or flow, these values
should be written with appropriate values. The register “Use default K” value
indicates to the device if the values in Velocity K value and Area are valid and
should be used. Set “Use default K” value to 0 in order to use values written to the
Velocity K value and Area registers.

Coils
The coil registers represent functions of the device. The value returned when
reading a coil register indicates the status of the last function execution. If the value
is 1, then the last time the function executed was a success. If the value is 0, then
the function has either not been executed since power on or failed during the last
execution. To execute a function, write 1 to the corresponding register. A response
will be returned immediately and the value of the coil will be set to 0. Once the
function completes, the value of the coil will be set to 1 if the operation was a
success. An application should poll the value of the coil periodically during this time
to determine if the function succeeded. If the coil value does not transition to 1 after
at most 10 seconds, then the operation failed.

Coil 1 – Zero function
The zero function will attempt to recalibrate the zero point. This may be needed if
the pressure sensor has drifted over time. Note that the zero function will only re-
zero the sensor if the current pressure is within ±2% of span pressure of the
previous zero. If the current pressure is outside the valid band, the zero function will
fail and the coil value will remain 0. If the sensor has drifted far enough that the zero
function fails, then pressure will have to be applied to bring the current pressure
closer to the current zero, and the zero function will have to be executed multiple
times until the actual zero is reached.

Coil 2 – Span function
The span function will attempt to recalibrate the maximum positive or negative
pressure. Note that accurate span pressure depends on an accurate zero pressure.
The span function will fail if the zero function has not been executed within the last
5 minutes.

Coil 3 – Reset factory Defaults function
The reset factory defaults function resets the zero, span, Velocity K value, Area,
and Use Default K Value variables back to their factory default values.

Coil 4 – Reset Device function
The reset device function allows this device to be reset remotely from Modbus

When the reset device function coil is written with a value of 1, the device will
immediately respond with success. The reset will take place approximately 5
seconds after the command was received. Writing the value 0 to this coil has no
effect.

Multi-address Support
Multi-Address support allows a register to be read or written to using different byte
orientations specified by the address range. For example, input register 0003 can
also be read at 2003, 4003 and 6003 with different byte orientations as listed in the
below table. Registers that do not have multi-address support are only available in
Big-Endian byte orientation (Modbus

Communication Protocol standard).

LeD Status
The device provides three LEDs to indicate status and activity. The LEDs are
located to the right of the terminal block and are arranged from left to right as green,
yellow, and red. The green LED indicates communication with the pressure sensor.
This LED will flash very quickly most of the time, even without requests from the
Modbus

interface. The yellow LED indicates Modbus

communication addressed

to this specific device. The frequency and duration of this LED depends on the baud
rate and size of request sent. The red LED indicates error states by flashing specific
error codes periodically.

MaINTeNaNCe/RePaIR
Upon final installation of the Series MS2 Differential Pressure Transmitter, no
routine maintenance is required. The Series MS2 is not field serviceable and it is
not possible to repair the unit. Field repair should not be attempted and may void
warranty.

WaRRaNTY/ReTURN
Refer to “Terms and Conditions of Sales” in our catalog and on our website. Contact
customer service to receive a Return Goods Authorization number before shipping
the product back for repair. Be sure to include a brief description of the problem
plus any additional application notes.

function Name
Read Coils
Read Holding Registers
Read Input Registers
Write Single Coil
Write Single Register
Write Multiple Registers

function Code
01
03
04
05
06
16

Register
0001

0002-0003

0004-0005

Description
Use default K
value. (Default
value: 1)
Velocity K
value
Area (ft

)

Data
Type
Unsigned
16 bit
integer
Float

Float

Value
0...1

0...9.999

0.01...999.99

Range
False-True

0...9.999

0.01...999.99

Multi-address
Supported
Yes

Yes

Register
0001

0002

0003

0004

Description
Perform Zero
function
Perform Span
function
Reset Factory
Defaults
Reset Device

Data
Type
Boolean

Boolean

Value
0...1

0...1

Range
False-True

False-True

Multi-address
Supported
No

Byte Order
Big-Endian
Byte Swap
Word Swap
Little-Endian

address Range
1 – 2000
2001 – 4000
4001 – 6000
6001 – 8000

float/32Bit Values
Register 1
MSB
A
B
C
D

B
A
D
C

Register 2

C
D
A
B

LSB
D
C
B
A

16Bit Values
Register 1
MSB
A
B
A
B

LSB
B
A
B
A

Number of flashes
Solid On (not flashing)
1
2

error Description
Invalid Factory Configuration
Invalid Modbus

Communication Protocol Address

Intelligent Serial Configuration in progress, unit may
not respond to requests
Communication with pressure sensor failed