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.