Category: Eaton

Users attempting to configure and scale analogue units via the Modbus master and

network may find considerable difficulty with this issue. If specifically designed

software is available for configuration and scaling of Modbus devices, this may be the

simplest and most convenient method of scaling and encoding data. An example

of this is the PCS83 software, which is available from Eaton’s MTL product line for

configuring the MTL838B-MBF. This makes encoding and scaling decisions

transparent to the user.

The difficulties of implementing an encoding and scaling regime for the MTL838B-MBF

via the Modbus host are discussed in depth on page 37. Users are strongly

recommended to read this section before selecting the configuration method to be

used with the MTL838B-MBF.

Data Encoding and Scaling

As has been mentioned earlier, an important area of the communication along the

network, that is not defined by the Modbus protocol, is the encoding of numerical data.

A related problem is the adoption of a scaling system for the data once it has been

encoded. (Note: this is an area which requires careful consideration by users of the

MTL838B-MBF.)

There is no problem here for manufacturers who are supplying complete systems,

based on the Modbus network, as they can select a data encoding and scaling system

appropriate to their needs. However, for manufacturers who are supplying products for

general use, there is no possibility that they will be able to determine which data

encoding system will be used by their customers, and they must allow the data

encoding technique to be user selectable.

Three data encoding techniques are the most popular – IEEE, 16-bit unsigned and 16

bit offset.

A further area of difficulty associated with the encoding of data is the way in which the

data is scaled – to provide a resolution of the measured value appropriate to the

requirements of  each application.

The message fields

The address field

Slave addresses may be in the range 1 to 247 with Modbus (1 to 255 with JBUS). A

slave is addressed by the master placing the relevant address in the address field of

the query message. When the slave sends its response, it places its own address in

the message field to indicate to the master that the correct slave is replying.

Address ‘0’ is used for ‘broadcast’ messages. All suitable slaves read them, but do not

provide responses to such query messages.

The function field

Function codes may be in the range 1 – 255. though not all functions will be supported

by all devices. When a message is sent from a master to a slave, the function code

defines the action that is required from the addressed slave. Examples of action

requested by the various function codes include: read input status; read register

content; change a status within the slave; etc..

When the slave sends its response to the master, it will repeat the function code

received, to indicate that the slave has understood the query and acted accordingly. If

the query instruction could not be carried out by the slave, an ‘exception response’ is

generated and the function code and data fields are used to inform the master of the

reason for the exception.

The exception response is generated by returning the original function code from the

master, but with its most significant bit set to ‘1’. Further information regarding the

exception response is passed to the master via the data field of the response

message. This tells the master what kind of error occurred and allows it to take the

most appropriate action – either to repeat the original message, to try and diagnose

what has happened to the slave, to set alarms or to take whatever action is most

appropriate.

Modbus message framing

Modbus messages must be structured (or ‘framed’) so that the different Modbus

components can detect the start, content structure and end point of a message. It also

allows any errors to be detected.

The framing used depends on the transmission mode chosen – ASCII  or RTU.

ASCII message framing

In ASCII mode, messages start with a ‘colon’ (:), which in hex is ‘3A’. The message

end is shown by ‘carriage return/line feed’ (CRLF) or ‘OD OA’ in hex .

The allowable characters for all other fields are hexadecimal 0-9. A-F. Networked

components monitor the bus continuously for the ‘colon’ character and when one is

received, they decode the next field (the address field) to find out if the address is for

that slave. If the address is for another slave, then no action is taken, and the slave

returns to monitoring for the ‘colon’ character. If  the field following the colon is the

address of the slave in question, then the slave continues to read the message and to

act on it’s contents.

Intervals of up to one second can elapse between characters within the message, but

if an interval is greater than this, then the device assumes that an error has occurred.

If the delay occurs in the ‘query’ to a slave, then the addressed slave will discard the

message received up to that point and wait till the next message (marked by the colon

start character) is received.

Slave, Transmitter and Input addressing The following discusses the allocation of addresses to the slaves on the Modbus network – including the MTL838C  – and the allocation of addresses for the transmitters and inputs connected to each MTL838C. Addressing MTL838C slaves Modbus allows slave addresses in the range 1 to 247.  JBUS allows slave addresses in the range 1 to 255.  This is the only difference between the two protocols.  Since the MTL838C can only have addresses in the range 1 to 31. it will work equally well with either protocol. The Modbus address for each MTL838C slave is set via the PC software.  For reasons of security, it is not possible to set the address of the slave via the Modbus host. The address for each RS485 port on the MTL838C may be set from 1 to 255.  This facility allows the MTL838C to be connected to the same master twice or to two different masters independently.  There is no restriction regarding simultaneous communication on both ports. The unit will respond via the port on which it received the query.

Slave, Transmitter and Input addressing The following discusses the allocation of addresses to the slaves on the Modbus network – including the MTL838C  – and the allocation of addresses for the transmitters and inputs connected to each MTL838C. Addressing MTL838C slaves Modbus allows slave addresses in the range 1 to 247.  JBUS allows slave addresses in the range 1 to 255.  This is the only difference between the two protocols.  Since the MTL838C can only have addresses in the range 1 to 31. it will work equally well with either protocol. The Modbus address for each MTL838C slave is set via the PC software.  For reasons of security, it is not possible to set the address of the slave via the Modbus host. The address for each RS485 port on the MTL838C may be set from 1 to 255.  This facility allows the MTL838C to be connected to the same master twice or to two different masters independently.  There is no restriction regarding simultaneous communication on both ports. The unit will respond via the port on which it received the query.

Off-line Configuration Off-line configuration requires the use of the PC software briefly described below. Once configured, the configuration parameters are stored in non-volatile memory within the MTL838C. The PC software By far the simplest method of configuring the MTL838C is using the PC software.  This software has been specifically designed to perform all of the complex calculations that must be carried out, in order to configure the unit.  These calculations are transparent to the user, and this method provides a convenient and time efficient method. Alternatively, as explained before, the master could read the configured parameters after initial off-line configuration and these may then be stored within the host for use in the event of a database failure. Interconnection of the MTL838C The MTL838C may be connected to a Modbus host in a number of ways—as was mentioned earlier it may be connected for multi-drop or point-to-point operation. Two RS485 ports, 1 and 2. are provided on the MTL838C. As there are two ports the unit can either be connected to a single Modbus master, with dual redundancy, or connected to two separate Modbus hosts. The MTL838C will respond on whichever RS485 connection the query is received, and there is no restriction placed on the simultaneous use of both interfaces.  The slave address for each RS-485 port is set using the PC Software.

On-line Configuration Configuring the unit via the Modbus master and the network might seem to be the simplest method at first sight, but there are a number of practical difficulties with this configuration technique.  This approach means that the user must deal with a number of complex aspects which require a significant investment of the configurer’s time before they are understood fully.  A further difficulty may be a lack of the necessary memory space within the Modbus master.  If the configuration is likely to be changed frequently it could even be necessary for the system designer to design specific ‘user interface’ screens, such as those used by the PC software, to allow changes to be made by operators.  This would be a time consuming and costly task. For most users, the attraction of being able to use the Modbus master to configure the unit is that the configuration can be re-sent if the slave’s memory becomes corrupted. Whilst this is true, it is not possible to avoid the difficulties (and costs) outlined earlier and the decision to adopt a strategy of configuring via the Modbus master should be arrived at only after due consideration. A cost effective compromise would be to perform the initial configuration via the PC software, and then read the configuration parameters stored in the MTL838C via the host.  The stored parameters could then be re-written to the MTL838C should the configuration database ever become corrupt. If a user intends to adopt the on-line configuration method, the calculation of configuration parameters for storage in the master can be simplified, and the possibility of ‘human error’ reduced, by using the PC software to input the required data and data format, and then reading the stored values (encoded correctly in the required data format) back from the MTL838C via Modbus.  The user should still realize that any subsequent alterations of the parameters will require further use of the PC software.

Configuring the MTL838C The MTL838C must first be configured using software on a PC and the USB connection.  This configures things such as the slave address and communication parameters.  After the initial configuration, the MTL838C is ready to communicate with the Modbus host.  At this point, the remaining configuration may be done in one of two ways: • on-line via the Modbus link, direct from the host • off-line using the PC software and USB connection Using the PC software is required for initial configuration and recommended for first time configuration of the measuring channels Off-line Configuration Off-line configuration requires the use of the PC software briefly described below. Once configured, the configuration parameters are stored in non-volatile memory within the MTL838C.

The analog-input multiplexer system The MTL838C is an analog multiplexer receiver that is used with the MTL831C hazardous area millivolt input multiplexer transmitter. The status of up to 32 analog inputs may be communicated from the hazardous area to the safe area via a data highway, comprised of a simple twisted pair – over distances up to 2km. Each data highway must be protected by an MTL5553/5053 digital isolator when the inputs are located in a Zone 0 or 1 hazardous area.  The MTL831C is typically used with thermocouple and RTD inputs and is intrinsically safe.  It can be mounted in a Zone 0 or 1 hazardous area and will accept 16 inputs.  For systems that do not require Zone 0 or Zone 1 installation, the MTL5553/5053 can be eliminated. Up to two MTL831C transmitters can be combined on a single MTL838C receiver input – up to a total of 32 analog inputs – as shown in Figure 1. The MTL838C acts as a Modbus slave . It may be connected into any standard Modbus network, with up to 31 MTL838C slaves on each network.  If each unit has its full complement of 32 analog inputs, the status of a total of 992 analog inputs may be passed to a Modbus master using a single RS485 network.