Siemens LMV 5 Series Technical Instructions Download

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LMV Series

Technical Instructions

Document No. LV5-1000

SCC Inc.

Page 19

Section 8

Important Modbus Notes

Operating

Mode

(Address 39)

Remote

Mode

(Address 43)

X62 Action

(Hard-wired)

Remote -> Local

(Address 41)

Modbus Control Action

IntLC Bus

0 (automatic)

Bumpless transition

to W1

Bumpless transition from W3

to W1

Burner controls to W3 setpoint

using internal load controller

1 (manual)

No transition to W1

but W1 is now

displayed as setpoint

Bumpless transition if a valid

W3 is already established

Burner modulates at

commanded firing rate without

load controller intervention

2 (burner off)

Transition to W1

Burner remains off

ExtLC Bus

0 (automatic)

Bumpless transition

to W1

Local/Remote is not

considered, remains at firing

rate in memory

Burner modulates at

commanded firing rate without

load controller intervention*

1 (manual)

2 (burner off)

Local/Remote is not

considered, remains off

Burner remains 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.

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