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4. MODBUS RTU Data Sets
4.1 Overview.
The NX6100/PPC6000 series controllers support the following MODBUS register types :
•Coils (1 to 19) - These registers can be written to by a MODBUS master to provide control of
NX6100/PPC6000 series controllers. In addition these registers can be read to interrogate the
last value written.
•Input registers (1 to 1200, 16 bit) - These can be read by a MODBUS master to monitor combus-
tion and other variables, from the NX6100/PPC6000 series controllers.
•Holding registers (1 to 1600, 16 bit) - These can be written to by a MODBUS master to change
modulation control of NX6100/PPC6000 series controllers. As with Coils, these registers can be
read to interrogate the last value written.
For daughterboard firmware levels 1.103 and greater, the MODBUS system can keep a maximum of
100 items (including read and written items) updated. Values will normally all refresh within one
second, but if more than 50 items are being updated, this will extend to two seconds.
Only the last 100 registers requested from the MODBUS master will be remembered by the
NX6100/PPC6000 controller. If more than 100 items are requested, the oldest items will cease to be
active. This means that if more than 100 different items have been accessed since a particular item
was read, this item may return zero the next time it is read, since the MODBUS system is no longer
tracking that item. It will then become active and will hold valid data on the next read. In practice
this is not an issue since there are nowhere near 100 items available from the NX6100/PPC6000
series controllers that will be of interest to plant operators etc. Most values that are available are
'engineers key' values that are provided for fault-finding and diagnostic purposes.
However, Coils and Holding registers are never 'dropped' from the item list in this way. So if the
burner is turned off via MODBUS (for example), then 100 or more different read items are requested
but the 'burner off' register is not updated, the burner will stay off.
The MODBUS 'read' functions listed in section 4.4 below allow multiple items to be read in one
MODBUS request. However the maximum size of MODBUS messages is limited to 128 bytes. The
maximum number of items that can be read in one message is 61, but it is recommended that signifi-
cantly fewer registers are read. Again, this many contiguous registers will not all contain useful data
anyway.
Note: Check daughterboard firmware level if Option 0.7 and 0.8 are correctly set and the
MODBUS message structure has been verified as correct.
All address references in the MODBUS messages use numbers relative to zero. For example, the first
holding register SETPOINT 1 CONTROL VALUE, would be 40001 and be referenced as register
0000. Similarly, coil 0008, BURNER ON/OFF, would be register 0007.
