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Modbus - Bird's Eye View

The most commonly used form of Modbus protocol is RTU over RS-485.

A Modbus RTU network has one Master and one or more Slaves. Each slave has a unique 8-bit device address or unit number. Packets sent by the master include the address of the slave the message is intended for. The slave must respond only if its address is recognized, and must respond within a certain time period or the master will call it a "no response" error.

Modbus data is most often read and written as "registers" which are 16-bit pieces of data. Most often, the register is either a signed or unsigned 16-bit integer. If a 32-bit integer or floating point is required, these values are actually read as a pair of registers. The most commonly used register is called a Holding Register, and these can be read or written. The other possible type is Input Register, which is read-only.

The exceptions to registers being 16 bits are the coil and the discrete input, which are each 1 bit only. Coils can be read or written, while discrete inputs are read-only. Coils are usually associated with relay outputs.

The type of register being addressed by a Modbus request is determined by the function code. The most common codes are 3 for "read holding registers", and may read 1 or more. Function code 6 is used to write a single holding register. Function code 16 is used to write one or more holding registers.

Modbus documentation is not standardized. Therefore, holding register #1 could be documented as address 0, register 1, or register 40001. See more explanation of notation in sections on this page.

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Modbus 101 - Introduction to Modbus

This page will not teach you everything about Modbus, but will attempt to answer some of the most frequently asked questions. If you still have questions about Modbus after reviewing this information, just click Support above.

What is Modbus?
Where do I start?
Review of Modbus Register Types
Review of Modbus Function Codes
Review of Modbus Exception (error) Codes

Modbus: When 40001 really means 1, or 0 really means 1
40001: Modicon convention notation for Modbus registers
If registers are 16-bits, how do I read Floating Point or 32-bit data?
What does notation like 40001:7 mean?
How to I read individual bits in a register?

Can I put 2 gateways on the same Modbus network?
How many devices can I have on a Modbus network?

What is the difference between Modbus RTU, TCP, and ASCII?
Where can I get a copy of the Modbus protocol specification?
Where can I find out about RS-485 wiring?

What is Modbus?

Modbus is an industrial protocol standard that has been in use for many years. Modbus ASCII and Modbus RTU are relatively simple serial protocols that use EIA-232 or EIA-485 to transmit data packets. The protocol defines function codes and the encoding scheme for transferring data as either single points (1-bit, coils) or as 16-bit data registers. This basic data packet is then encapsulated according to the protocol specifications for Modbus ASCII, RTU, or TCP.

The TCP version of Modbus follows the OSI Network Reference Model; however the serial implementations of Modbus do not. Modbus/TCP defines the presentation and application layers in the OSI model.

Modbus protocol is defined as a master/slave protocol, meaning a device operating as a master will poll one or more devices operating as a slave. This means a slave device cannot volunteer information; it must wait to be asked for it. The master will write data to a slave device’s registers, and read data from a slave device’s registers. A register address or register reference is always in the context of the slave’s registers.

Modbus/TCP makes the definition of master and slave less obvious because Ethernet allows peer to peer communication. The definition of client and server are better known entities in Ethernet based networking. In this context, the slave becomes the server and the master becomes the client. There can be more than one client obtaining data from a server. In Modbus terms, this means there can be multiple masters as well as multiple slaves. Rather than defining master and slave on a physical device by device basis, it now becomes the system designer’s responsibility to create logical associations between master and slave functionality.

Where do I start?

Here are the first few things you need to find out:

(1) What is the physical connection?

Modbus RTU uses RS-485 or RS-232. Modbus TCP uses Ethernet. If you are looking for a Control Solutions gateway, you will need to pick the model that matches the electrical interface of the equipment you want to connect. If you are choosing a Control Solutions I/O device, pick one that matches your network.

(2) How are the registers mapped?

When using a gateway to interface a Modbus device to a non-Modbus network, you need to get documentation from the equipment manufacturer that describes the available registers and how to address them. Modbus protocol does not provide a means for registers to automatically identify themselves. Control Solutions cannot determine this information for you. You must consult the equipment manufacturer.

When using Control Solutions I/O devices (such as AddMe III) you will find this information in the online help files that came with the device, or on our web site.

(3) What are the communication parameters?

Modbus RTU requires that you know or define baud rate, character format (8 bits no parity, etc), and slave ID (aka slave address, unit number, unit ID). A mis-match in any of these will result in no communication.

Modbus TCP requires that you know or define IP addresses on the network. In some cases, you also need unit ID's. Control Solutions Modbus TCP devices may use the unit ID, or may ignore it, depending on the device and the application.

Review of 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.

Control Solutions gateways will support all register types when the gateway is the master, or when operating in direct mode (Babel Buster SP-GW). Control Solutions gateways that connect a non-Modbus device to a Modbus network use only holding registers to represent the non-Modbus device data (e.g. copy LonWorks network variable data to holding registers).

Most Control Solutions I/O devices use only holding registers for all types of inputs and outputs. This convention originated with the introduction of configurable hardware that could map any type of input or output to the same register.

Review of 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.

Modbus Function Codes Recognized by CSI Gateways
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

Review of 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:

1
Illegal Function The function code received in the query is not recognized by the slave or is not allowed by the slave.
2
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.
3
Illegal Data Value The value contained in the query's data field is not acceptable to the slave.
4
Slave Device Failure An unrecoverable error occurred while the slave was attempting to perform the requested action
6
Slave Device Busy The slave is engaged in processing a long-duration command. The master should try again later.
10
Gateway Path Unavailable Specialized use in conjunction with gateways, usually means the gateway is misconfigured or overloaded
11
Gateway Target Device Failed to Respond Specialized use in conjunction with gateways, indicates no response was received from the target device.

Modbus: When 40001 really means 1, or 0 really means 1

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.

40001: 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-IDA 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.

Note: Only the LonWorks versions of Babel Buster gateways use this notation as short hand to conserve CP space. Many other equipment manufacturers still use this convention in their products. Depending on which combination of products you are using, you may have to translate between Modicon and current conventions.

The types of registers referenced in Modbus devices, and supported by Babel Buster gateways, 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 originally supported by Babel Buster gateways was 0 to 9999. The extended range addressing was later added to all new Babel Buster products that use this notation.

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 Babel Buster (LonWorks) gateways 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 (also referred to as register number 1). The reference 40001 will appear in documentation and is used to define the Modbus register in the location property of the functional block in a LonWorks gateway. The address 0000 will be transmitted in the message packet. Addresses are often not directly used by the application or the user.

On occasion, it is necessary to access more than 10,000 of a register type. Based on the original convention, there is another defacto standard that looks very similar. Additional register types and reference ranges recognized by Babel Buster (LonWorks) gateways are as follows:

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

When using the extended register referencing, it is mandatory that all register references be exactly six digits. This is the only way Babel Buster will know the difference between holding register 40001 and coil 40001. If coil 40001 is the target, it must appear as 040001.

If registers are 16-bits, how do I 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 gateways, support a "swapped" option. This means you simply check the "swapped" option if the other device treats wide data in the opposite order relative to Control Solutions default order.

Control Solutions Modbus products all default to placing the high order register first, or in the lower numbered register. This is known as "big endian", and is consistent with Modbus protocol which is by definition big endian.

What does notation like 40001:7 mean?

This is a commonly used notation for referencing individual bits in a register. This particular example references 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?

The bit mask shown in the expanded form of the RTU read 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 notation of register number followed by a colon and number from 0 to 15 indicates a single bit picked from that register. The hex bit mask values would be as follows, assuming a register number of 40001.

40001:0 mask: 0001
40001:1 mask: 0002
40001:2 mask: 0004
40001:3 mask: 0008
40001:4 mask: 0010
40001:5 mask: 0020
40001:6 mask: 0040
40001:7 mask: 0080
40001:8 mask: 0100
40001:9 mask: 0200
40001:10 mask: 0400
40001:11 mask: 0800
40001:12 mask: 1000
40001:13 mask: 2000
40001:14 mask: 4000
40001:15 mask: 8000

Sometimes a 16-bit register is used to hold two 8-bit values. To strip bytes using the bit mask, you would enter the following:

Low byte mask: 00FF
High byte mask: FF00

What is the difference between Modbus RTU, TCP, and ASCII?

Modbus protocol defines a Protocol Data Unit (PDU) that is independent of the underlying communication layers. Modbus RTU is the most commonly used, and is a binary representation of the PDU with addressing before the PDU, a CRC appended to the end.  Modbus ASCII is a representation of the same PDU using all printable characters (and generally twice as many bytes). Modbus TCP is essentially the exact same PDU as Modbus RTU, except the CRC is left off of the application layer byte string and left up to the TCP layer to deal with automatically. There are also some additional addressing bytes in the TCP encapsulation of the RTU packet.

The function codes, register numbering and addressing are identical regardless of protocol variant. Register types are the same, i.e, the same data blocks are defined for the Modbus device.

Can I put 2 gateways on the same Modbus network?

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

Multiple gateways 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 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.

Where can I get a copy of the Modbus protocol specification?

You can get a copy of the Modbus protocol specification by visiting www.modbus.org. There are three documents of primary interest: (1) The application layer protocol which defines the function codes and addressing; (2) The definition of use over a serial line (RTU); (3) The definition of use over Ethernet (TCP). You will be asked to accept terms of use, but there is no cost for these specifications.