Modbus

Modbus is the most common protocol used for communicating with industrial hardware. It works by one 'Master' station, often a PC, requesting data from one or more 'slave' stations.

Originally invented as a 'Serial Port' protocol, Modbus over Ethernet is fast becoming the new 'standard'.

Hexatec software fully supports the Modbus protocol, over Ethernet and Serial connections, including Remote modems. It can also support both Master and Slave modes. This flexibility ensures it may be used with a wide range of Modbus communication equipment.

Master / Slave Functionality

Modbus is a Master / Slave protocol. A number of devices may be interconnected using the protocol, but only one device can be the master. Communication is always instigated and controlled by the master device. All other devices are connected as slaves and will only communicate in response to requests or commands from the single master. Slaves are each configured to have a unique address. Modbus TCP is a variant of the original standard and allows concurrent master / slave operation.

Master Functionality

Saturn (or SCAN1000) issues Modbus commands in sequence on a per network basis and waits a predetermined time for a reply from the addressed slave device. If, for any reason, an incorrect or no response is returned, the process is repeated a set number of times. This ensures communications can be maintained even if interruptions, such as electrical noise, occur.

Connection dependent settings that need to be defined:

	Serial Port. Port number and associated settings. ASCII / RTU selection.
	Modbus TCP. A network device (or IP address) and Port number (502 default).
	Remote (modem) connections. A TAPI device is selected and a number to dial included. ASCII / RTU selection.

For device reads, Saturn supports:

	Coil Status
	Input Status
	Holding Registers
	Input Registers

For device writes, Saturn uses Single Coil Writes for boolean tags and Single Register Writes for analogue tags. Modbus does not support text or string operations.

Some variation is possible in the way Register data is interpreted, so a number of options are included:

Registers can be handles as:

	2 Byte
	2 Byte Signed
	4 Byte
	4 Byte Signed
	4 Byte Float

and options allow for byte and word order. In this way, special forms of Modbus, such as: Daniels, Enron, etc., are supported.

Slave Functionality

A choice of Serial Port or Network connection is available. If remote access via a modem is required, this should be connected via a Serial Port and configured separately.

Settings are a subset of the equivalent Master options described above.

Multiple Network Support

The SCAN1000 / Saturn software supports multiple networks at the same time. Allowing data to be collected from Multiple networks concurrently, even if one is Serial and another Ethernet.

The software can also work as a Modbus slave, which means that the PC can also aggregate data and make it available to another Modbus master system such as a central DCS.

The diagram shows some of these possibilities.

For example it is possible to:

Configure a PC running Saturn (or SCAN1000) to be the master device to one or more networks of slave devices. Each slave network will use one of the three connection options described above.
Connect a Saturn (or SCAN1000) to another Modbus master, as a Slave.

This allows great flexibility in how various Modbus communicating devices may be connected.

The configuration of each network is completely separate and there is no interaction.

It is even possible to use the software as a protocol converter between dissimilar Modbus devices or between Modbus and other protocols.

This flexibility offers great scope to the range of possibilities, when used for real world applications.

Protocol Versions (RTU, ASCII)

Modbus devices can communicate using either of the two versions of the Modbus protocol, RTU or ASCII. RTU is a binary format requiring the least number of characters for transmission, whereas ASCII uses only text readable characters requiring more per message and is thus slower. Some devices will only support one or the other. All connected devices should be configured to the same protocol version.

Modbus TCP does not use the ASCII option.

Connection Methods

Both SCAN1000v5 and Saturn software support three different methods of connecting to Modbus communication devices:

	Serial connections use a dedicated serial cable connection between a master and one or more slave devices. A specific standard is used such as RS232 and all connected devices should support and be configured for the same standard.
	Modbus TCP uses a network connection for communications. This can either be dedicated, or shared with other network traffic. Modbus TCP is now the world's leading industrial Ethernet protocol in terms of the number of units shipped.
	Remote connections use modem technology normally connected to the telephone system to access remote devices.

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