3.6 Interconnection of the MTL838C
The MTL838C may be connected to a Modbus host in a number of ways.
Two RS485 outputs, Link 1 and Link 2, are provided on the MTL838C. As there are
two outputs 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
Modbus addresses for each Link may only be set by using the PC software.
3.7 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 several 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.
Also, when the PC software is communicating with the MTL838C, Modbus
communications are disabled. This is to prevent the possibility of writing to the
configuration database simultaneously from two sources.
3.8 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.
3.8.1 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. The
MTL838C will accept addresses in the range 1 to 255 for each Link. Care should be
taken when setting the addresses for each Link using the PC software. For example,
if both Links are connected to the same Host, the addresses should probably be
different.
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.
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