Modbus register addressing
Pay close attention to the following terms in vendors Modbus documentation
Coil Status, or Modbus function code 1**
o ALC register address range: Leading 0 (zero) = 1-65535*
o Read / Write
Input Status, or Modbus function code 2**
o ALC register address range: Leading 1 (one) = 100001-165535*
o Read only
Holding Register, or Modbus function code 3**
o ALC register address range: Leading 4 (four) = 400001-465535*
o Read / Write
Input Register, or Modbus function code 4**
o ALC register address range: Leading 3 (three) = 300001-365535*
o Read only
*Some vendors document their registers starting with zero in the least significant bit and some start with one like ALC does. If you find a vendor using 0 for their first register you must add one to all of their documented registers to arrive at the number ALC needs to read that point.
**There are many more function codes in the Modbus protocol that define things like read multiple registers, write a register or a reply from a device. They are outside the scope of this document because the ALC driver handles these function codes in the background.
The Modbus register address (sometimes called register, address, code, or parameter) must be a decimal value. If the register addresses in the points list include the letters A-F, use a scientific calculator to convert these hexadecimal values to decimal values. For example, J-Bus can be read with a Modbus driver.
If the register addresses in the points list are 5-digit numbers in the range #0001-#9999 (where # = the prefix 1, 3, or 4), you may use the addresses as they are, or change them to fit the formats in the table above by shifting the prefix one digit to the left. For example, if you see the address 38129, you can also use 308129.
Modbus register types
Register types define how the information received or transmitted is interpreted.
Input Or Holding Registers
UINT = Unsigned Integer = 16bit register resulting in a decimal number of 0-65,535 or 0xFFFF.
UINT32 = Unsigned Integer = 32bit register resulting in a decimal number of 0 to 4,294,967,295 or 0xFFFF,FFFF. In terms of ModBus registers this type occupies two registers, the number addressed and the next highest register which will go unmentioned.
SINT = Signed Integer = 16bit register resulting in a decimal number of -32,768 to 32768 (in binary the first bit is the sign).
SINT32 = Signed Integer = 32bit register resulting in a decimal number of -2,147,483,647 to 2,147,483,648 (in binary the first bit is the sign). In terms of ModBus registers this type occupies two registers, the number addressed and the next highest register which will go unmentioned.
FLOAT = Value with a decimal point. In terms of ModBus registers this type occupies two registers, the number addressed and the next highest register which will go unmentioned. The two registers used are not a set standard and you may have to make use of the Reverse FLOAT/32 Bit Integer feature in the LGR driver if you find you are getting bad FLOAT data from a vendor.
BIT# = One binary bit contained in a register where the bit # is specified by 0-15.
Coil registers
DO = Coil / Discrete (binary) output
Input Registers
DI = Discrete (binary) input
Note: Offsets, dividers or scaling:
Rather than dealing with FLOAT registers many vendors will define a certain offset or scaling necessary to read their information.
Example: vendor transmits a temperature of 7653 and documents a scaling of 100 so this must be divided by 100 to arrive at 76.53°F.
Formatting the ALC ModBus URL
Protocol:
o modbus:// This might be port S1 or S2 on an LGR, see the driver protocols, ModBus page
o modbus1:// Specifically port S1, if enabled, see the driver protocols, ModBus page
o modbus2:// Specifically port S2, if enabled, see the driver protocols, ModBus page
o mtcpip:// Modbus over IP
Register Type, see register types page:
o UINT
o SINT
o UINT32
o SINT32
o FLOAT
o BIT#
o DO
o DI
ModBus Register address, see Register address page:
o 1-65535
o 100001-165535
o 400001-465535
o 300001-365535
Device Address
o Slave or server device address 1-247
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