You can only send the command over the active input. To switch away from VGA to HDMI, you must send the command over the VGA connection. To switch back to VGA from HDMI, you must send the command over the HDMI connection.

Tried sending the command from VGA connection to switch to HDMI but still failed.

On the PC with the VGA connection, open DDM and adjust brightness - if you can do that, then the command should work. If it doesn't, save the DDM diagnostic report: while holding down the SHIFT key, right-click on the DDM system tray icon and and then select "Save diagnostic report" from the popup menu. Then post the content of the report here

21:13:34.344...Video power down timeout = 600

21:13:36.505...Display powered ON

21:13:36.561...Attempting to restore window layout at 3286,1123...non-existant

21:13:46.590...0ld=CMN1476000000030000 GSM5A34000000030100

21:13:46.591...New=CMN1476000000030000 DELD057000000030100

21:13:46.591...HPD event detected... pausing

21:13:51.598...Changed display configuration

21:13:51.603...Starting enumeration

21:13:51.605......Skipping unsupported device MONITOR\CMN1476\{4d36e96e-e325-11ce-bfc1-08002be10318}\0001

21:13:51.615...2. DELD057.MDCK254K0GST on Intel(R) HD Graphics Family

21:13:51.615......Monitor assignment = 1

21:13:51.616......Adapter name = \\.\DISPLAY2

21:13:51.616......Monitor name = DELL S2340L(Analog)

21:13:51.616......Vendor ID = 8086

21:13:51.662......GPU driver = igdumdim64 20.19.15.4331

21:13:51.662......Flags = 00000001.00000003

21:13:51.662......Display tag = CF020E56

21:13:51.663......Monitor number = 2

21:13:51.663......Monitor handle = 0004063B

21:13:51.663......Raw EDID = 00FFFFFFFFFFFF0010AC57D054534730111901030E331D78EAE295A2554F9F26115054A54B00714F8180D1C001010101010101010101023A801871382D40582C4500FD1E1100001E000000FF004D44434B3235344B304753540A000000FC0044454C4C2053323334304C0A20000000FD00384C1E5311000A20202020202000CF

21:13:53.498......Error: GetVCP10h: 0xC0262585 = The data received from the monitor is invalid

21:13:53.499......VCP10 = FFFFFFFF

21:13:53.499......OS device handle = 0004063B.00000000

21:13:54.302......Error: failed to obtain capabilities string

21:13:54.302......Error: failed to obtain output mask from display driver

21:13:56.858......VCP10 = 00640028

21:13:56.858......OS device handle = 0004063B.00000000

21:13:56.859......Capabilities string = (prot(monitor)type(lcd)model(s2340l)cmds(01 02 03 07 0c e3 f3)vcp(02 04 05 06 08 10 12 14(05 08 0b 0c) 16 18 1a 52 60(01 03) ac ae b2 b6 c8 c9 d6(01 04 05) dc(00 02 03 05 ) f0(00 01) df e0 e1 e2(00 01 02 04 06 0e 12 14) f1 f2)mccs_ver(2.1)mswhql(1)) (cached)

21:13:57.084......Configuration = 10001.FFFF0063.1911

21:13:57.085......Model = Dell S2340L

21:13:57.085......Model year = FY13

21:13:57.086......Capabilities = (prot(monitor)type(lcd)model(s2340l)cmds(01 02 03 07 0c e3 f3)vcp(02 04 05 06 08 10 12 14(05 08 0b 0c) 16 18 1a 52 60(01 03) ac ae b2 b6 c8 c9 d6(01 04 05) dc(00 02 03 05 ) f0(00 01) df e0 e1 e2(00 01 02 04 06 0e 12 14) f1 f2)mccs_ver(2.1)mswhql(1)) (cached)

21:13:57.086......Opcodes supported = 02,04,05,06,08,10,12,14,16,18,1A,52,60,AC,AE,B2,B6,C8,C9,D6,DC,F0,DF,E0,E1,E2,F1,F2

21:13:57.195......Timing = 67.50 kHz (-), 60.00 Hz (-)

21:13:57.195...Ending enumeration

21:13:57.199...0ld=CMN1476000000030000 DELD057000000030100

21:13:57.199...New=CMN1476000000030000 DELD057000000030100

21:13:57.209...Attempting to restore window layout at 3286,1123...non-existant

21:13:57.209...Creating VCP table on DELD057.MDCK254K0GST

21:13:57.268......Opcode 0x12 = 00640032

21:13:57.377......Opcode 0xC8 = 00005605

21:13:59.149......Error: GetVCPE0h: 0xC0262589 = An operation failed because a DDC/CI message had an invalid value in its command field

21:14:00.918......Opcode 0xE0 = 00010000

21:14:01.028......Opcode 0xE1 = 00010000

21:14:01.138......Opcode 0xE2 = 00FF0000

21:14:02.441...Starting enumeration

21:14:02.443......Skipping unsupported device MONITOR\CMN1476\{4d36e96e-e325-11ce-bfc1-08002be10318}\0001

21:14:02.448...2. DELD057.MDCK254K0GST on Intel(R) HD Graphics Family

21:14:02.449......Monitor assignment = 1

21:14:02.449......Adapter name = \\.\DISPLAY2

21:14:02.449......Monitor name = DELL S2340L(Analog)

21:14:02.449......Vendor ID = 8086

21:14:02.451......GPU driver = igdumdim64 20.19.15.4331

21:14:02.451......Flags = 00000001.00000003

21:14:02.451......Display tag = CF020E56

21:14:02.451......Monitor number = 2

21:14:02.451......Monitor handle = 0004063B

21:14:02.452......Raw EDID = 00FFFFFFFFFFFF0010AC57D054534730111901030E331D78EAE295A2554F9F26115054A54B00714F8180D1C001010101010101010101023A801871382D40582C4500FD1E1100001E000000FF004D44434B3235344B304753540A000000FC0044454C4C2053323334304C0A20000000FD00384C1E5311000A20202020202000CF

21:14:02.518......VCP10 = 00640028

21:14:02.562......OS device handle = 0004063B.00000000

21:14:02.594......Capabilities string = (prot(monitor)type(lcd)model(s2340l)cmds(01 02 03 07 0c e3 f3)vcp(02 04 05 06 08 10 12 14(05 08 0b 0c) 16 18 1a 52 60(01 03) ac ae b2 b6 c8 c9 d6(01 04 05) dc(00 02 03 05 ) f0(00 01) df e0 e1 e2(00 01 02 04 06 0e 12 14) f1 f2)mccs_ver(2.1)mswhql(1)) (cached)

21:14:03.251......Configuration = 10001.FFFF0063.1911

21:14:03.259......Model = Dell S2340L

21:14:03.261......Model year = FY13

21:14:03.261......Capabilities = (prot(monitor)type(lcd)model(s2340l)cmds(01 02 03 07 0c e3 f3)vcp(02 04 05 06 08 10 12 14(05 08 0b 0c) 16 18 1a 52 60(01 03) ac ae b2 b6 c8 c9 d6(01 04 05) dc(00 02 03 05 ) f0(00 01) df e0 e1 e2(00 01 02 04 06 0e 12 14) f1 f2)mccs_ver(2.1)mswhql(1)) (cached)

21:14:03.264......Opcodes supported = 02,04,05,06,08,10,12,14,16,18,1A,52,60,AC,AE,B2,B6,C8,C9,D6,DC,F0,DF,E0,E1,E2,F1,F2

21:14:03.399......Timing = 67.50 kHz (-), 60.00 Hz (-)

21:14:03.404...Ending enumeration

21:14:03.412...0ld=CMN1476000000030000 DELD057000000030100

21:14:03.413...New=CMN1476000000030000 DELD057000000030100

21:14:03.425...Attempting to restore window layout at 3286,1123...non-existant

21:14:03.426...Creating VCP table on DELD057.MDCK254K0GST

21:14:03.817......Opcode 0x12 = 00640032

21:14:03.934......Opcode 0xC8 = 00005605

21:14:04.047......Opcode 0xE0 = 00010000

21:14:04.157......Error: GetVCPE1h: 0xC026258B = An error occurred because the checksum field in a DDC/CI message did not match the message's computed checksum value. This error implies that the data was corrupted while it was being transmitted from a monitor to a computer

21:14:04.267......Opcode 0xE1 = 00010000

21:14:04.382......Opcode 0xE2 = 00FF0000

21:14:06.673...0ld=CMN1476000000030000 DELD057000000030100

21:14:06.674...New=CMN1476000000030000 DELD057000000030100

21:14:06.674...Attempting to restore window layout at 3286,1123...non-existant

21:14:06.674...Changed display settings

21:14:06.736...0ld=CMN1476000000030000 DELD057000000030100

21:14:06.737...New=CMN1476000000030000 DELD057000000030100

21:14:06.737...Attempting to restore window layout at 3286,1123...non-existant

21:14:06.737...Changed display settings

21:17:12.586...DecControl 10 = Ok

21:17:14.224...DecControl 10 = Ok

21:17:14.550...DecControl 10 = Ok

21:17:14.807...DecControl 10 = Ok

21:17:15.040...DecControl 10 = Ok

21:17:15.297...DecControl 10 = Ok

21:17:15.698...DecControl 10 = Ok

21:17:16.108...DecControl 10 = Ok

21:17:16.329...DecControl 10 = Ok

21:17:16.558...DecControl 10 = Ok

21:17:16.963...DecControl 10 = Ok

21:17:17.202...DecControl 10 = Ok

21:17:17.644...DecControl 10 = Ok

21:17:17.989...DecControl 10 = Ok

21:17:18.382...DecControl 10 = Ok

21:17:18.689...DecControl 10 = Error

21:17:18.926...DecControl 10 = Error

21:17:19.168...DecControl 10 = Error

21:17:19.628...DecControl 10 = Error

21:17:19.988...DecControl 10 = Error

21:17:22.122...DecControl 10 = Error

21:18:01.683...1:SetActiveInput hdmi = Error

21:18:27.723...\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

21:18:27.723...Dump: complete Windows device enumeration

21:18:27.723...1.DeviceName      = \\.\DISPLAY1

21:18:27.723.....DeviceFlags     = 00000005

21:18:27.723.....DeviceString    = Intel(R) HD Graphics Family

21:18:27.724.....DeviceID        = PCI\VEN_8086&DEV_0A16&SUBSYS_397817AA&REV_09

21:18:27.724.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{DD54D160-8ED0-40EB-8E64-9B6EF3093D91}\0000

21:18:27.724.....a.MonitorName   = \\.\DISPLAY1\Monitor0

21:18:27.725.......MonitorFlags  = 00000003

21:18:27.725.......MonitorString = Generic PnP Monitor

21:18:27.725.......MonitorID     = MONITOR\CMN1476\{4d36e96e-e325-11ce-bfc1-08002be10318}\0001

21:18:27.725.......MonitorKey    = \Registry\Machine\System\CurrentControlSet\Control\Class\{4d36e96e-e325-11ce-bfc1-08002be10318}\0001

21:18:27.725.......Raw EDID      = 00FFFFFFFFFFFF000DAE76140000000033150103801F11780A2B359757548F2923505400000001010101010101010101010101010101F71B5692500015302C1D340035AE10000018000000FE004E3134304247452D4C42320A20000000FE00434D4E0A202020202020202020000000FE004E3134304247452D4C42320A200069

21:18:27.726...2.DeviceName      = \\.\DISPLAY2

21:18:27.726.....DeviceFlags     = 00000001

21:18:27.726.....DeviceString    = Intel(R) HD Graphics Family

21:18:27.726.....DeviceID        = PCI\VEN_8086&DEV_0A16&SUBSYS_397817AA&REV_09

21:18:27.727.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{DD54D160-8ED0-40EB-8E64-9B6EF3093D91}\0001

21:18:27.727.....a.MonitorName   = \\.\DISPLAY2\Monitor0

21:18:27.727.......MonitorFlags  = 00000003

21:18:27.727.......MonitorString = DELL S2340L(Analog)

21:18:27.728.......MonitorID     = MONITOR\DELD057\{4d36e96e-e325-11ce-bfc1-08002be10318}\0000

21:18:27.728.......MonitorKey    = \Registry\Machine\System\CurrentControlSet\Control\Class\{4d36e96e-e325-11ce-bfc1-08002be10318}\0000

21:18:27.728.......Raw EDID      = 00FFFFFFFFFFFF0010AC57D054534730111901030E331D78EAE295A2554F9F26115054A54B00714F8180D1C001010101010101010101023A801871382D40582C4500FD1E1100001E000000FF004D44434B3235344B304753540A000000FC0044454C4C2053323334304C0A20000000FD00384C1E5311000A20202020202000CF

21:18:27.729...3.DeviceName      = \\.\DISPLAY3

21:18:27.729.....DeviceFlags     = 00000000

21:18:27.729.....DeviceString    = Intel(R) HD Graphics Family

21:18:27.729.....DeviceID        = PCI\VEN_8086&DEV_0A16&SUBSYS_397817AA&REV_09

21:18:27.730.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{DD54D160-8ED0-40EB-8E64-9B6EF3093D91}\0002

21:18:27.730...4.DeviceName      = \\.\DISPLAY4

21:18:27.730.....DeviceFlags     = 00000000

21:18:27.731.....DeviceString    = NVIDIA GeForce GT 720M  

21:18:27.731.....DeviceID        = PCI\VEN_10DE&DEV_1140&SUBSYS_380017AA&REV_A1

21:18:27.731.....DeviceKey       = \Registry\Machine\System\CurrentControlSet\Control\Video\{61881BCD-E967-4BB9-AEE2-B6CAA35BC0B7}\0000

21:18:27.731...Total number of monitors enumerated = 2

21:18:27.732...\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

21:18:27.732...Dumping VCP table on DELD057.MDCK254K0GST

21:18:27.789......Opcode 0x02 = 00020001

21:18:27.898......Opcode 0x10 = 00640000

21:18:28.006......Opcode 0x12 = 0064004B

21:18:28.114......Opcode 0x14 = 000C0005

21:18:28.223......Opcode 0x16 = 00640064

21:18:28.332......Opcode 0x18 = 00640064

21:18:28.441......Opcode 0x1A = 00640064

21:18:28.550......Opcode 0x52 = 00FF0000

21:18:28.658......Opcode 0x60 = 000E0001

21:18:28.766......Opcode 0xAC = 00010680

21:18:28.875......Opcode 0xAE = 00001752

21:18:28.984......Opcode 0xB2 = 00080001

21:18:29.092......Opcode 0xB6 = 00050003

21:18:29.201......Opcode 0xC8 = 00005605

21:18:29.309......Opcode 0xC9 = FFFF0201

21:18:29.418......Opcode 0xD6 = 00FF0001

21:18:29.528......Opcode 0xDC = 00FF0000

21:18:29.636......Opcode 0xDF = 00FF0201

21:18:29.745......Opcode 0xE0 = 00010000

21:18:29.855......Opcode 0xE1 = 00010000

21:18:29.966......Opcode 0xE2 = 00FF0000

21:18:30.076......Opcode 0xF0 = 00FF0000

21:18:30.186......Opcode 0xF1 = 00010001

21:18:30.294......Opcode 0xF2 = 00010000

21:18:30.347......VCP table dumped

It is working now. Thanks!