Siemens LMV5 Series Technical Instructions page 289

LMV Series
Important Modbus Notes
Operating Remote
Mode Mode
(Address 39) (Address 43)
0 (automatic)
IntLC Bus
1 (manual)
2 (burner off)
0 (automatic)
1 (manual)
ExtLC Bus
2 (burner off)
*A value of 1 must be written to address 43 to turn the burner on if a value of 2 was previously written to address 43 to turn the burner off.
Manual Operation
Manual operation from the AZL takes priority over any Modbus operation, although "SetLoad" is
common to the remote firing rate. If a previous firing rate or "SetLoad" is established in memory, it will
be used when manual or remote firing rate operation is commanded.
ExtLC Bus Caution
In operating mode ExtLC Bus, a communication loss will cause the burner to stay in the last commanded
state. If the burner was firing, it will restart at the firing rate in memory. If the burner was commanded
off, operation will not resume unless communication is re-established or the operating mode is changed.
Displayed Setpoint
When in ExtLC Bus, setpoint is displayed as "ext LC" since it is not used. When in IntLC Bus and providing
a remote firing rate, the W3 setpoint is displayed (even if it is 0) although it is not used. It is important
to send a valid setpoint for display purposes as well as for bumpless transfer to W1 after a
communication timeout or switch to Local mode since a transition from firing rate control to W3
setpoint control will occur for at least one scan before assigning control to W1. If W3 is less than the
actual setpoint plus the "off" hysteresis, the burner will shut down before transferring to W1. This is not
necessary when going from Local to Remote, but would be good practice.
Sequence of Control
Local to Remote: W1 setpoint control -> remote firing rate control
Remote to Local (or communication timeout): remote firing rate control -> W3 setpoint control (very
briefly) -> W1 setpoint control
Considerations When Using Remote Firing Rate Control
When the internal load controller is not used to provide the firing rate, functions such as the thermostat
function and cold start function will not work. However, these should still be configured for use in Local
mode. In Remote mode, it is important to duplicate the functionality when determining the firing rate.
An additional temperature sensor connected to X60 for cold start can be read via Modbus address 83 if
a Pt1000 or Ni1000 RTD is used.
SCC Inc.
X62 Action
(Hard-wired)
Bumpless transition Bumpless transition from W3
to W1
No transition to W1 Bumpless transition if a valid
but W1 is now W3 is already established
displayed as setpoint
Local/Remote is not
considered, remains at firing
Bumpless transition
to W1
Local/Remote is not
considered, remains off
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Remote -> Local
(Address 41)
Burner controls to W3 setpoint
to W1 using internal load controller
commanded firing rate without
load controller intervention
Transition to W1
commanded firing rate without
rate in memory load controller intervention*
Technical Instructions
Document No. LV5-1000
Modbus Control Action
Burner modulates at
Burner remains off
Burner modulates at
Burner remains off
Section 8