Thermistors read constant, even if disconnected

Controllers Smoothie
1

My thermistors always read this, even if I disconnected all three thermistors from Smoothieboard. What is the problem? Thank you

M105

ok T:122.1 /0.0 @0 T1:122.2 /0.0 @0 B:131.9 /0.0 @0

M105

ok T:122.1 /0.0 @0 T1:122.2 /0.0 @0 B:131.9 /0.0 @0

M105

ok T:122.3 /0.0 @0 T1:122.3 /0.0 @0 B:132.1 /0.0 @0

M105

ok T:122.3 /0.0 @0 T1:122.3 /0.0 @0 B:132.1 /0.0 @0

  1. Hotend temperature control configuration

temperature_control.hotend.enable true # Whether to activate this ( “hotend” ) module at all.
temperature_control.hotend.thermistor_pin 0.24 # Pin for the thermistor to read
temperature_control.hotend.heater_pin 2.7 # Pin that controls the heater, set to nc if a readonly thermistor is being defined
temperature_control.hotend.thermistor Semitec #
temperature_control.hotend.set_m_code 104 #
temperature_control.hotend.set_and_wait_m_code 109 #
temperature_control.hotend.designator T #

  1. Hotend2 temperature control configuration

temperature_control.hotend2.enable true # Whether to activate this ( “hotend” ) module at all.
temperature_control.hotend2.thermistor_pin 0.25 # Pin for the thermistor to read
temperature_control.hotend2.heater_pin 1.23 # Pin that controls the heater
temperature_control.hotend2.thermistor Semitec #
temperature_control.hotend2.set_m_code 104 #
temperature_control.hotend2.set_and_wait_m_code 109 #
temperature_control.hotend2.designator T1 #

temperature_control.bed.enable true #
temperature_control.bed.thermistor_pin 0.23 #
temperature_control.bed.heater_pin 2.5 #
temperature_control.bed.thermistor Honeywell100K #
temperature_control.bed.set_m_code 140 #
temperature_control.bed.set_and_wait_m_code 190 #
temperature_control.bed.designator B #

Imported from wikidot

2

This looks like the thermistors are absent even when connected.
Could you try connecting a 10K resistor across one of the thermistor inputs, and see what it reads ?

3

Thank you for your response!

When I bridged it with an 11 K ohm resistor, my voltmeter read 2.18V.

(When T0 is open, the voltmeter reads 3.11V)

The output from M105 did not change, however.

4

Pulled the board out of my printer today to put under scope.

When plugged into USB for power, the V across T0 (while T0 is open) is 3.04V. Is this typical?

I flashed the latest edge build date Apr 28,2015. There is nothing plugged into Smoothieboard except USB. I reset the config file to the original. I had a strange result:

With hotend.thermistor_pin = 0.23,
M105 responds T:125.6 /0.0 @0 B:129.1 /0.0 @0

When I modified config to set hotend.thermistor_pin = 0.26 and reset,
M105 responds T:250.1 /0.0 @0 B:220.1 /0.0 @0

Could this be indicative of any particular problem or broken components?