Appendix C. Modbus Protocol Reference

Modbus Register Types

The types of registers referenced in Modbus devices include the following:
• Coil (Discrete Output)
• Discrete Input
• Input Register
• Holding Register

Whether a particular device includes all of these register types is up to the manufacturer. It is very common to find all I/O mapped to holding registers only. Coils are 1-bit registers, are used to control discrete outputs, and may be read or written. Discrete Inputs are 1-bit registers used as inputs, and may only be read. Input registers are 16-bit registers used for input, and may only be read. Holding registers are the most universal 16-bit register, may be read or written, and may be used for a variety of things including inputs, outputs, configuration data, or any requirement for "holding" data.

Modbus Function Codes

Modbus protocol defines several function codes for accessing Modbus registers. There are four different data blocks defined by Modbus, and the addresses or register numbers in each of those overlap. Therefore, a complete definition of where to find a piece of data requires both the address (or register number) and function code (or register type).

The function codes most commonly recognized by Modbus devices are indicated in the table below. This is only a subset of the codes available - several of the codes have special applications that most often do not apply.

Function Code Register Type
1 Read Coil
2 Read Discrete Input
3 Read Holding Registers
4 Read Input Registers
5 Write Single Coil
6 Write Single Holding Register
15 Write Multiple Coils
16 Write Multiple Holding Registers

Modbus Exception (error) Codes

When a Modbus slave recognizes a packet, but determines that there is an error in the request, it will return an exception code reply instead of a data reply. The exception reply consists of the slave address or unit number, a copy of the function code with the high bit set, and an exception code. If the function code was 3, for example, the function code in the exception reply will be 0x83. The exception codes will be one of the following:

Illegal Function The function code received in the query is not recognized by the slave or is not allowed by the slave.
Illegal Data Address The data address (register number) received in the query is not an allowed address for the slave, i.e., the register does not exist. If multiple registers were requested, at least one was not permitted.
Illegal Data Value The value contained in the query's data field is not acceptable to the slave.
Slave Device Failure An unrecoverable error occurred.
Slave Device Busy The slave is engaged in processing a long-duration command. The master should try again later.
(hex 0A)
Gateway Path Unavailable Gateway could not establish communication with target device.
(hex 0B)
Gateway Target Device Failed to Respond Specialized use in conjunction with gateways, indicates no response was received from the target device.
(hex 11)
Gateway Target Device Failed to Respond No response from slave, request timed out.

Modicon convention notation for Modbus registers

Modbus was originally developed by Gould-Modicon, which is presently Schneider Electric. The notation originally used by Modicon is still often used today, even though considered obsolete by present Modbus standards. The advantage in using the Modicon notation is that two pieces of information are included in a single number: (a) The register type; (b) The register number. A register number offset defines the type.

The types of registers referenced in Modbus devices, and supported by i.Report, include the following:
• Coil (Discrete Output)
• Discrete Input
• Input Register
• Holding Register

Valid address ranges as originally defined for Modbus were 0 to 9999 for each of the above register types. Valid ranges allowed in the current specification are 0 to 65,535. The address range applies to each type of register, and one needs to look at the function code in the Modbus message packet to determine what register type is being referenced. The Modicon convention uses the first digit of a register reference to identify the register type. 

Register types and reference ranges recognized by i.Report are as follows:

0x = Coil = 00001-09999
1x = Discrete Input = 10001-19999
3x = Input Register = 30001-39999
4x = Holding Register = 40001-49999

Translating references to addresses, reference 40001 selects the holding register at address 0000, most often referred to as holding register number 1. The reference 40001 will appear in documentation using Modicon notation, but i.Report requires specifying "holding register" and entering that register number as just "1".

On occasion, it was necessary to access more than 10,000 of a register type using Modicon notation. Based on the original convention, there is another defacto standard that looks very similar. Additional register types and reference ranges recognized by i.Report are as follows:

0x = Coil = 000001-065535
1x = Discrete Input = 100001-165535
3x = Input Register = 300001-365535
4x = Holding Register = 400001-465535

If registers are 16-bits, how does one read Floating Point or 32-bit data?

Modbus protocol defines a holding register as 16 bits wide; however, there is a widely used defacto standard for reading and writing data wider than 16 bits. The most common are IEEE 754 floating point, and 32-bit integer. The convention may also be extended to double precision floating point and 64-bit integer data.

The wide data simply consists of two consecutive "registers" treated as a single wide register. Floating point in 32-bit IEEE 754 standard, and 32-bit integer data, are widely used. Although the convention of register pairs is widely recognized, agreement on whether the high order or low order register should come first is not standardized. For this reason, many devices, including all Control Solutions Modbus devices, support register "swapping". This means you simply check the "swapped" option (aka "High reg first" in some devices) if the other device treats wide data in the opposite order relative to Control Solutions default order.

What does notation like 40001:7 mean?

This is a commonly used notation for referencing individual bits in a register. This particular example, 40001:7, references (Modicon) register 40001, bit 7. Bits are generally numbered starting at bit 0, which is the least significant or right most bit in the field of 16 bits found in a Modbus register.

How do I read individual bits in a register?

Documentation tends to be slightly different for every Modbus device. But if your device packs multiple bits into a single holding register, the documentation will note up to 16 different items found at the same register number or address. The bits may be identified with “Bn” or “Dn” or just “bit n”. Most of the time, the least significant bit will be called bit 0 and the most significant will be bit 15. It is possible you could find reference to bit 1 through bit 16, in which case just subtract one from the number to reference the table below.

You cannot read just one bit from a holding register. There is no way to do that - Modbus protocol simply does not provide that function. You must read all 16 bits, and then test the individual bit you are interested in for true or false (1 or 0). The i.Report devices provide an automatic way of doing that by including a "mask" in each object map. Each time the register is read, the mask will be logically AND-ed with the data from the register, and the result will be right justified to yield a 1 or 0 based on whether the selected bit was 1 or 0. The i.Report provides optimization when successive object maps are selecting different bits from the same register. The Modbus register will be read from the slave once, and the 16-bit data will be shared with successive maps, with each object map selecting its bit of interest.

The bit mask shown in the i.Report object map is a 4 digit hexadecimal (16 bit) value used to mask out one or more bits in a register. The selected bits will be right justified, so a single bit regardless of where positioned in the source register will be stored locally as 0 or 1. The hex bit mask values would be as follows:

    B0/D0/bit 0 mask = 0001
    B1/D1/bit 1 mask = 0002
    B2/D2/bit 2 mask = 0004
    B3/D3/bit 3 mask = 0008
    B4/D4/bit 4 mask = 0010
    B5/D5/bit 5 mask = 0020
    B6/D6/bit 6 mask = 0040
    B7/D7/bit 7 mask = 0080
    B8/D8/bit 8 mask = 0100
    B9/D9/bit 9 mask = 0200
    B10/D10/bit 10 mask = 0400
    B11/D11/bit 11 mask = 0800
    B12/D12/bit 12 mask = 1000
    B13/D13/bit 13 mask = 2000
    B14/D14/bit 14 mask = 4000
    B15/D15/bit 15 mask = 8000

Some Modbus devices also back two 8-bit values into a single 16-bit register. The two values will typically be documented as “high byte” and “low byte” or simply have “H” and “L” indicated. If you run into this scenario, the masking for bytes is as follows:

    High byte mask = FF00
    Low byte mask = 00FF

When the mask value in the i.Report is more than just one bit, the mask is still logically AND-ed with the data from the Modbus slave, and the entire resulting value is right justified to produce an integer value of less than the original bit width of the original register.

There have been a few instances of documenting packed bits in a 32-bit register. Although Modbus protocol is strictly 16-bit registers, some implementations force you to read pairs of registers. If your device documents 32 packed bits, then you would insert 0000 in front of each mask above, and the remainder of the list would be as follows:

    B16/D16/bit 16 mask = 00010000
    B17/D17/bit 17 mask = 00020000
    B18/D18/bit 18 mask = 00040000
    B19/D19/bit 19 mask = 00080000
    B20/D20/bit 20 mask = 00100000
    B21/D21/bit 21 mask = 00200000
    B22/D22/bit 22 mask = 00400000
    B23/D23/bit 23 mask = 00800000
    B24/D24/bit 24 mask = 01000000
    B25/D25/bit 25 mask = 02000000
    B26/D26/bit 26 mask = 04000000
    B27/D27/bit 27 mask = 08000000
    B28/D28/bit 28 mask = 10000000
    B29/D29/bit 29 mask = 20000000
    B30/D30/bit 30 mask = 40000000
    B31/D31/bit 31 mask = 80000000

Deciphering Modbus Documentation

Documentation for Modbus is not well standardized. Actually there is a standard, but not well followed when it comes to documentation. You will have to do one or more of the following to decipher which register a manufacturer is really referring to:

a) Look for the register description, such as holding register, coil, etc. If the documentation says #1, and tells you they are holding registers, then you have holding register #1. You also have user friendly documentation.

b) Look at the numbers themselves. If you see the first register on the list having a number 40001, that really tells you register #1, and it is a holding register. This form of notation is often referred to as the old Modicon convention.

c) Look for a definition of function codes to be used. If you see a register #1, along with notation telling you to use function codes 3 and 16, that also tells you it is holding register #1.

IMPORTANT: Register 1 is address 0. Read on…

d) Do the numbers in your documentation refer to the register number or address? Register #1 is address zero. If it is not clear whether your documentation refers to register or address, and you are not getting the expected result, try plus or minus one for register number. All Control Solutions products refer to register numbers in configuration software or web pages. However, some manufacturers document their devices showing address, not register numbers. When you have addresses, you must add one when entering that register into configuration software from Control Solutions.

Can I put 2 i.Reports on the same Modbus network?

You can not have more than one Master on a Modbus RTU (RS-485) network. Therefore, if the i.Report is to be configured as the Master, you can only have 1 i.Report. You cannot use multiple i.Report devices to read more points from the same Modbus slave device.

Multiple i.Report devices configured as slaves can reside on the same Modbus RS-485 network.

If you are using RS-232 devices, you can have only two devices total, regardless of how they are configured. RS-232 is not multi-drop.

How many devices can I have on a Modbus RTU network?

Logically you can address over 250 devices; however, the RS-485 transceivers are not capable of physically driving that many devices. Modbus protocol states that the limit is 32 devices, and most RS-485 transceivers will agree with this. Only if all devices on the network have low load transceivers can you have more than 32 devices.

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