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INM MTL83xC PC Modbus Rev 4
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.
Modbus TCP (Ethernet) does not allow multiple slaves on the same IP address. For
this reason the address becomes somewhat unimportant except that it is still used.
The address on the MTL838C-MBT which is set using the PC software must match
what the Master is using.
3.8.2 Addressing the transmitters of each MTL838C
Each MTL831C transmitter accepts up to 16 sensor inputs and there can be one
or two MTL831C transmitters connected to a single MTL838C. The address of
the MTL831C as seen by the MTL838C is determined by whether a jumper wire
is installed on the MTL831C. A jumper wire not installed gives the MTL831C an
address of ‘1’ and a jumper installed gives it an address of ‘2’. The following shows
the sensor numbers used by the MTL838C for a given MTL831C address.
MTL831C Address Sensor Numbers
1 0 - 15
1 32 CJC
2 16 - 31
2 33 CJC
Addressing of the MTL831C transmitters affects which sensor is given which
address in the MTL838C. For example, if only one MTL831C is connected to the
MTL838C but its address jumper is installed, it will be at transmitter address ‘2’ and
the sensor range will be 16 – 3, 1, 33.
It is also important that with two MTL831Cs connected to a single MTL838C, that
one and only one of them has the jumper installed. Otherwise they will both be at
the same address and communication between the MTL831s and the MTL838C
will fail. Likewise, there can be no more than two MTL831Cs connected to a single
MTL838C.
CJC stands for Cold Junction Compensation and reports the average temperature
of the MTL831C circuit board. This is an average of two temperature sensors
and should not be used by the Modbus Host to cold junction compensate
thermocouples. Instead the thermocouple channels should be compensated by the
MTL831C by selecting the correct Input Type.
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