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Chapter 4. Serial Communications
108 PanaFlow™ MV80, MV82 & MV84 User’s Manual
4.2.6 Menu Items
The following menu items are in the Output Menu and allow selection and control of the Modbus communication
protocol.
4.2.6.1 Address
When the Modbus protocol is selected, the Modbus address is equal to the user programmable device address if it is
in the range 1…247, in accordance with the Modbus specification. If the device address is zero or is greater than 247,
then the Modbus address is internally set to 1.
4.2.6.2 Comm Protocol
The Comm Protocol menu allows selection of “Modbus RTU Even,” “Modbus RTU Odd,” “Modbus RTU None2” or
“Modbus RTU None1” (non-standard Modbus) with Even, Odd and None referring to the parity selection. When even or
odd parity is selected, the unit is configured for 8 data bits, 1 parity bit and 1 stop bit; with no parity, the number of stop
bits is 1 (non-standard) or 2. When changing the protocol, the change is made as soon as the
ENTER
key is pressed.
4.2.6.3 Modbus Units
The Modbus Units menu is used to control the display units, where applicable, for the meter's variables. The internal
base units of the meter are: °F, psia, lbm/sec, ft
3
/sec, Btu/sec, lbm/ft
3
. The display variables are displayed in the user
selected display units.
4.2.6.4 Modbus Order
The byte order within registers and the order in which multiple registers containing floating point or long integer data
are transmitted may be changed with this menu item. According to the Modbus specification, the most significant
byte of a register is transmitted first, followed by the least significant byte. The Modbus specification does not
prescribe the order in which registers are transmitted when multiple registers represent values longer than 16 bits.
Using this menu item, the order in which registers representing floating point or long integer data and/or the byte
order within the registers may be reversed for compatibility with some PLCs and PC software.
Table 4 below lists the four selections are available in this menu. When selecting an item, the protocol is changed
immediately without having to press the
ENTER
key.
Table 4: Modbus Order Options
Order Description
0-1:2-3 Most significant register first, most significant byte first (default)
2-3:0-1 Least significant register first, most significant byte first
1-0:3-2 Most significant register first, least significant byte first
3-2:1-0 Least significant register first, least significant byte first
All of the registers are affected by the byte order, including strings and registers representing 16-bit integers. The
register order affects only the order of those registers representing 32-bit floating point and long integer data, but
does not affect single 16-bit integers or strings.