Eaton MTL838C-MBF MTL fieldbus networks

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.

Initialization mode

The MTL838C has two distinct modes of operation  - normal and initialization.

It will always enter initialization mode during power-up.  It can also be triggered by

the detection of internal hardware or software faults, or after receiving an instruction

from the host to reset some or all of the configuration registers.

During initialization, the unit will ignore all commands from the master.

The initialization period will take 1 or 2 seconds to complete all the necessary

operations and calculations.  Following successful initialization, the unit will

automatically enter, or return to, normal operation mode.

If a corrupted configuration database is detected during initialization the unit will

revert to a set of default values, and on entering normal operation mode, will issue

exception responses when requested by the host to read input values.  Exception

responses will continue to be issued until the unit is re-configured.  The need to re

configure the unit will remain even if the MTL838C is powered down and back up.

If a corrupted configuration is detected, the slave address may be reset.  If this

occurs, the user must use the PC software to set the slave address.

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.