APC's smart protocol


The APC UPS protocol was originally analyzed by Pavel Korensky with additions from Andre H. Hendrick beginning in 1995, and we want to give credit for good, hard work, where credit is due. After having said that, you will see that Steven Freed built much of the orginal apcupsd information file. [Comment inserted by Riccardo Facchetti]

The start of this chapter of the apcupsd manual in HTML format was pulled from the Network UPS Tools (NUT) site. It has been an invaluable tool in improving apcupsd, and I consider it the Bible of APC UPS programming. In the course of using it, I have added information gleaned from apcupsd and information graciously supplied by APC. Hopefully, the additions made herein can benefit the original author and his programming project, and maybe some day, the Apcupsd project and the NUT project can join forces.


Here's the information on the elusive APC smart signaling protocol used by their higher end units (Back-UPS Pro, Smart-UPS, Matrix-UPS, etc). What you see here has been collected from a variety of sources. Some people analyzed the chatter between PowerChute and their hardware. Others sent various characters to the UPS and figured out what the results meant.

RS-232 differences

Normal 9 pin serial connections have TxD on 3 and RxD on 2. APC's smart serial ports put TxD on pin 1 and RxD on pin 2. This means you go nowhere if you use a normal straight through serial cable. In fact, you might even power down the load if you plug one of those cables in. This is due to the odd routing of pins - DTR and RTS from the PC usually wind up driving the on/off line. So, when you open the port, they go high and *poof* your computer dies.

Originally this evil hack was used to connect the UPS to the PC when this page was first being built. As you can see, I cheated and neglected the ground (only 2 wires!) and it still worked. This method can be used for playing around, but for professional systems this is obviously not a viable option.

That hack didn't work out so well (damned cats), so it was retired quite awhile back. The most practical solution was to go out and BUY the DOS/Win version of PowerChute just for the black (smart) cable. I recommend doing the same thing if you actually care about this thing working properly. Of course, if you have one of the newer packages that came with PowerChute, you already have the cable you need.

Diagram for cable hackers

If you are handy with cable creation tools, check out the 940-0024C clone diagram. That's the black "smart" cable normally provided with APC models sold after 1996. The loopback pins on that diagram are used to keep PowerChute happy by allowing cable detection. If you use the NUT apcsmart driver, those pins don't matter.

Many thanks to Steve Draper for providing this scan.

For additional information on cables, see the cables section of this manual.

The Smart Protocol

Despite the lack of official information from APC, this table has been constructed. It's standard RS-232 serial communications at 2400 bps/8N1. Don't rush the UPS while transmitting or it may stop talking to you. This isn't a problem with the normal single character queries, but it really does matter for multi-char things like "@000". Sprinkle a few calls to usleep() in your code and everything will work a lot better.

The following table describes the single character Code or command that you can send to the UPS, its meaning, and what sort of response the UPS will provide. Typically, the response shown below is followed by a newline (\n in C) and a carriage return (\r in C). If you send the UPS a command that it does not recognize or that is not available on your UPS, it will normally respond by "NA" for not available, otherwise the response is given in the "Typical results" column.

CodeMeaningTypical results
^AModel stringSMART-UPS 700
^NTurn on UPS (send twice, with > 1.5s delay between chars)
Only on 3rd gen SmartUPS and Black Back-UPS Pros
^ZPermitted EEPROM ValuesA large string (254 chars) that gives the EEPROM permitted values for your model. For details see below.
AFront panel testLight show + "OK" (and 2s beep)
BBattery voltageRanges - typical "27.87"
CInternal temperature (degrees C)Ranges - typical "036.0"
DRuntime calibration - runs until battery is below 25% (35% for Matrix)
This updates the 'j' values - only works at 100% battery charge
Can be aborted with a second "D"
! when on battery, $ on line
EAutomatic self test intervalsDefault = 336 (336 hours = 14 days) (336=14 days, 168=7 days, ON=power on, OFF=never)
FLine frequency, Hz60.00 (50.0 in Europe)
GCause of transferR = unacceptable utility voltage rate of change,
H = high utility voltage,
L = low utility voltage,
T = line voltage notch or spike,
O = no transfers yet (since turnon),
S = transfer due to serial port U command or activation of UPS test from front panel,
NA = transfer reason still not available (read again).
K--KShutdown with grace period (set with 'p') - need > 1.5s between first and second K Matrix/3rd gen SmartUPS/Black Back-UPS Pros: "OK", all others: "*"
LInput line voltageRanges - typical "118.3" or "228.8" in Europe
MMaximum line voltage received since last M queryRanges - typical "118.9" or "230.1" in Europe
NMinimum line voltage received since last N queryRanges - typical "118.9" or "226.2" in Europe
OOutput voltageRanges - typical "118.3" or "228.8" in Europe
PPower load % Ranges - typical "011.4" depends on what you have plugged in.
QStatus flagsBitmapped, see below
RTurn dumb
Only on 3rd gen SmartUPS, SmartUPS v/s, BackUPS Pro
SSoft shutdown after 'p' delay, return online when power returns
Only works when UPS is on battery
USimulate power failure!! when switching to battery, then $ when back on line
VOld firmware revision"GWD" or "IWI" The last character indicates the locale (Domestic, International).
WSelf test (battery), results stored in "X""OK"
XResults of last self test"OK" - good battery, "BT" - failed due to insufficient capacity, "NG" - failed due to overload, "NO" - no results available (no test performed in last 5 minutes)
YEnter smart mode"SM"
Z--ZShutdown immediately (no delay) - need > 1.5s between first and second Z N/A
aShow protocol version.alert messages.valid commands (delimited by periods) "3.!$%+?=#|.^A^N^Z+-789<@ABCDEFGKLMNOPQRSUVWXYZ'abcefgjklmnopqrsuvzy~^?" - Link-Level.alert-messages.commands
bFirmware revision"50.9.D" - 50 = SKU (variable length), 9 = firmware revision, D = country code (D=USA, I=International, A=Asia, J=Japan, M=Canada)
cUPS local idUPS_IDEN (you can program any 8 characters here)
eReturn threshold% battery charge threshold for return (00=00%, 01=15%, 02=25%, 03=90%)
fBattery level %Ranges - typical "100.0" when fully charged as should normally be the case
gNominal battery voltage (not actual voltage - see B) "012" or "024" or "048".
hMeasure-UPS: ambient humidity (%)"nnn.n" - percentage
iMeasure-UPS: dry contacts10 = contact 1, 20 = 2, 40 = 3, 80 = 4
jEstimated runtime at current load (minutes)"0112:" (note, it is terminated with a colon)
kAlarm delay0(zero) = 5 second delay after fail, T = 30 second delay, L = alarm at low battery only, N = no alarm
lLow transfer voltageDefault "103" or "208" in Europe
mManufacturing dateUnique within groups of UPSes (production runs)
nSerial numberUnique for each UPS
oNominal Output VoltageThe Nominal Output Voltage when running on batteries. Default "115" or "230" in Europe.
pShutdown grace delay, secondsDefault "020" (020/180/300/600)
qLow battery warning, minutesDefault "02"
rWakeup delay (time) - secondsDefault "000" (000/060/180/300)
sSensitivity"H" - highest, "M" - medium, "L" - lowest, "A" - autoadjust (Matrix only)
uUpper transfer voltageDefault "132" or "253" in Europe
tMeasure-UPS: ambient temperature (degrees C)"nn.nn"
xLast battery changeEight characters. Varies typically dd/mm/yy - 31/12/99
yCopyright notice"(C) APCC" - only works if firmware letter (from "V") is later than O
zReset the EEPROM to factory settings (but not ident or batt replacement date)
Not on SmartUPS v/s or BackUPS Pro
+Capability cycleCycle forward through possible values ("|" from UPS afterward to confirm change). Do not use this unless you know how to program your UPS EEPROM or you may damage your UPS.
-Capability cycleCycle backward through possible values ("|" from UPS afterward to confirm change)Do not use this unless you know how to program your UPS EEPROM or you may damage your UPS.
@nnnShutdown (after delay 'p') with delayed wakeup of nnn tenths of an hour (after 'r' time)Matrix/3rd gen UPS: "OK", others "*"
0x7f (DEL key)Abort shutdown - use to abort @, S, K--K"OK"
~Register #1See below
'Register #2See below
7Dip switch positions (if applicable)See below
8Register #3See below
9Line quality"FF" acceptable, "00" unacceptable
>Number of external battery packs attachedSmartCell models: "nnn" where nnn is how many external packs are connected
Non-SmartCell units: whatever has been set with >+ and >- by the user
Matrix UPS (and possibly Symmetra) specific commands
^Run in bypass modeIf online, "BYP" is received as bypass mode starts
If already in bypass, "INV" is received and UPS goes online
"ERR" received if UPS is unable to transfer
<Number of bad battery packs"nnn" - count of bad packs connected to the UPS
/Load current"nn.nn" - true RMS load current drawn by UPS
\Apparent load power"nnn.nn" - output load as percentage of full rated load in VA.
^VOutput voltage selection (editable)"A" - automatic according to input tap, "M" - 208 VAC, "I" - 240 VAC
^LFront panel language"E" - English, "F" - French, "G" - German, "S" - Spanish, "1" "2" "3" "4" - ?
wRun time conservation"NO" (disabled) or "02" "05" "08" - minutes of runtime to leave in battery (UPS shuts down "early")

Dip switch info

Bit Switch Option when bit=1
04Low battery alarm changed from 2 to 5 mins. Autostartup disabled on SU370ci and 400
13Audible alarm delayed 30 seconds
22Output transfer set to 115 VAC (from 120 VAC) or to 240 VAC (from 230 VAC)
31UPS desensitized - input voltage range expanded
4-7-Unused at this time

Status bits

This is probably the most important register of the UPS, which indicates the overall UPS status. Some common things you'll see:
  • 08 = On line, battery OK
  • 10 = On battery, battery OK
  • 50 = On battery, battery low
  • SM = Status bit is still not available (retry reading)
BitHex BitMeaning
00x011 = Runtime calibration occurring
Not reported by Smart UPS v/s and BackUPS Pro
10x021 = SmartTrim
Not reported by 1st and 2nd generation SmartUPS models
20x041 = SmartBoost
30x081 = On line (this is the normal condition)
40x101 = On battery
50x201 = Overloaded output
60x401 = Battery low
70x801 = Replace battery

Alert messages

These single character messages are sent by the UPS any time there is an Alert condition. All other responses indicated above are sent by the UPS only in response to a query or action command.
! Line Fail - sent when the UPS goes on-battery, repeated every 30 seconds until low battery condition reached. Sometimes occurs more than once in the first 30 seconds.
$ Return from line fail - UPS back on line power, only sent if a ! has been sent.
% Low battery - Sent to indicate low battery, but not on SmartUPS v/s or BackUPS Pro models
+ Return from low battery - Sent when the battery has been recharged to some level only if a % has been sent previously
? Abnormal condition - sent for conditions such as "shutdown due to overload" or "shutdown due to low battery capacity". Also occurs within 10 minutes of turnon.
= Return from abnormal condition - Sent when the UPS returns from an abnormal condition where ? was sent, but not a turn-on. Not implemented on SmartUPS v/s or BackUPS Pro models.
* About to turn off - Sent when the UPS is about to switch off the load. No commands are processed after this character is sent. Not implemented on SmartUPS v/s, BackUPS Pro, or 3rd generation SmartUPS models.
# Replace battery - Sent when the UPS detects that the battery needs to be replaced. Sent every 5 hours until a new battery test is run or the UPS is shut off. Not implemented on SmartUPS v/s or BackUPS Pro models.
& Check alarm register for fault (Measure-UPS) - sent to signal that temp or humidity out of set limits. Also sent when one of the contact closures changes states. Sent every 2 minutes, stops when the alarm conditions are reset. Only sent for alarms enabled with I. Cause of alarm may be determined with J. Not on SmartUPS v/s or BackUPS Pro.
| Variable change in EEPROM - Sent whenever any EEPROM variable is changed. Only supported on Matrix UPS and 3rd generation SmartUPS models.

Register 1

All bits are valid on the Matrix UPS. SmartUPS models only support bits 6 and 7. Other models do not respond.

BitHex BitMeaning
00x01In wakeup mode (typically lasts < 2s)
10x02In bypass mode due to internal fault - see register 2 or 3
20x04Going to bypass mode due to command
30x08In bypass mode due to command
40x10Returning from bypass mode
50x20In bypass mode due to manual bypass control
60x40Ready to power load on user command
70x80Ready to power load on user command or return of line power

Register 2

Matrix UPS models report bits 0-5. SmartUPS models only support bits 4 and 6. SmartUPS v/s and BackUPS Pro report bits 4, 6, 7. Unused bits are set to 0. Other models do not respond.

0Fan failure in electronics, UPS in bypass
1Fan failure in isolation unit
2Bypass supply failure
3Output voltage select failure, UPS in bypass
4DC imbalance, UPS in bypass
5Command sent to stop bypass with no battery connected - UPS still in bypass
6Relay fault in SmartTrim or SmartBoost
7Bad output voltage

Register 3

All bits are valid on the Matrix UPS and 3rd generation SmartUPS models. SmartUPS v/s and BackUPS Pro models report bits 0-5. All others report 0-4. State change of bits 1,2,5,6,7 are reported asynchronously with ? and = messages.

0Output unpowered due to shutdown by low battery
1Unable to transfer to battery due to overload
2Main relay malfunction - UPS turned off
3In sleep mode from @ (maybe others)
4In shutdown mode from S
5Battery charger failure
6Bypass relay malfunction
7Normal operating temperature exceeded

Interpretation of the Old Firmware Revision

The Old Firmware Revision is obtained with the "V" command, which gives a typical response such as "GWD" or "IWI", and can be interpreted as follows:

 Old Firmware revision and model ID String for SmartUPS & MatrixUPS

 This is a three character string XYZ

     where X == Smart-UPS or Matrix-UPS ID Code.
       range 0-9 and A-P
         1 == unknown
         0 == Matrix 3000
         5 == Matrix 5000
       the rest are Smart-UPS and Smart-UPS-XL
         2 == 250       3 == 400       4 == 400
         6 == 600       7 == 900       8 == 1250
         9 == 2000      A == 1400      B == 1000
         C == 650       D == 420       E == 280
         F == 450       G == 700       H == 700XL
         I == 1000      J == 1000XL    K == 1400
         L == 1400XL    M == 2200      N == 2200XL
         O == 3000      P == 5000

     where Y == Possible Level of Smart Features, unknown???
         G == Stand Alone
         T == Stand Alone
		 V == ???
         W == Rack Mount

     where Z == National Model Use Only Codes
         D == Domestic        115 Volts
         I == International   230 Volts
         A == Asia ??         100 Volts
         J == Japan ??        100 Volts

Interpretation of the New Firmware Revision

The New Firmware Revision is obtained with the "b" command, which give a typical response such as "50.9.D" or "60.11.I", and can be interpreted as follows:

 New Firmware revison and model ID String in NN.M.L is the format

      where NN == UPS ID Code.
          12 == Back-UPS Pro 650
          13 == Back-UPS Pro 1000
          52 == Smart-UPS 700
          60 == SmartUPS 1000
          72 == Smart-UPS 1400

          where NN now Nn has possible meanings.
              N  == Class of UPS
              1n == Back-UPS Pro
              5n == Smart-UPS
              7n == Smart-UPS NET

               n == Level of intelligence
              N1 == Simple Signal, if detectable WAG(*)
              N2 == Full Set of Smart Signals
              N3 == Micro Subset of Smart Signals

      where M == Possible Level of Smart Features, unknown???
          1 == Stand Alone
          8 == Rack Mount
          9 == Rack Mount

      where L == National Model Use Only Codes
          D == Domestic        115 Volts
          I == International   230 Volts
          A == Asia ??         100 Volts
          J == Japan ??        100 Volts
          M == North America   208 Volts (Servers)


Upon sending a ^Z, your UPS will probably spit back approximately 254 characters something like the following (truncated here for the example):

#uD43132135138129uM43229234239224uA43110112114108 ....

It looks bizarre and ugly, but is easily parsed. The # is some kind of marker/ident character. Skip it. The rest fits this form:

  • Command character - use this to select the value
  • Locale - use 'b' to find out what yours is (the last character), '4' applies to all
  • Number of choices - '4' means there are 4 possibilities coming up
  • Choice length - '3' means they are all 3 chars long
Then it's followed by the choices, and it starts over.

Matrix-UPS models have ## between each grouping for some reason.

Here is an example broken out to be more readable:

   u   D   4   3   127 130 133 136
   u   M   4   3   229 234 239 224
   u   A   4   3   108 110 112 114
   u   I   4   3   253 257 261 265
   l   D   4   3   106 103 100 097
   l   M   4   3   177 172 168 182
   l   A   4   3   092 090 088 086
   l   I   4   3   208 204 200 196
   e   4   4   2   00   15  50  90
   o   D   1   3   115
   o   J   1   3   100
   o   I   1   3   230 240 220 225
   o   M   1   3   208
   s   4   4   1     H   M   L   L
   q   4   4   2    02  05  07  10
   p   4   4   3   020 180 300 600
   k   4   4   1     0   T   L   N
   r   4   4   3   000 060 180 300
   E   4   4   3   336 168  ON OFF
   CMD == UPSlink Command.
         u = upper transfer voltage
         l = lower transfer voltage
         e = return threshold
         o = output voltage
         s = sensitivity
         p = shutdown grace delay
         q = low battery warning
         k = alarm delay
         r = wakeup delay
         E = self test interval

   DFO == (4)-all-countries (D)omestic (I)nternational (A)sia (J)apan
          (M) North America - servers.
   RSP == Total number possible answers returned by a given CMD.
   FSZ == Max. number of field positions to be filled.
   FVL == Values that are returned and legal.

Programming the UPS EEPROM

There are at this time a maximum of 12 different values that can be programmed into the UPS EEPROM. They are:

1.cThe UPS Id or name
2.xThe last date the batteries were replaced
3.uThe Upper Transfer Voltage
4.lThe Lower Transfer Voltage
5.eThe Return Battery Charge Percentage
6.oThe Output Voltage when on Batteries
7.sThe Sensitivity to Line Quality
8.pThe Shutdown Grace Delay
9.qThe Low Battery Warning Delay
10.kThe Alarm Delay
11.rThe Wakeup Delay
12.EThe Automatic Self Test Interval

The first two cases (Ident and Batt date) are somewhat special in that you tell the UPS you want to change the value, then you supply 8 characters that are saved in the EEPROM. The last ten item are programmed by telling the UPS that you want it to cycle to the next permitted value.

In each case, you indicate to the UPS that you want to change the EEPROM by first sending the appropriate query command (e.g. "c" for the UPS ID or "u" for the Upper Transfer voltage. This command is then immediately followed by the cycle EEPROM command or "-". In the case of the UPS Id or the battery date, you follow the cycle command by the eight characters that you want to put in the EEPROM. In the case of the other ten items, there is nothing more to enter.

The UPS will respond by "OK" and approximately 5 seconds later by a vertical bar (|) to indicate that the EEPROM was changed.


The apcupsd has a rather long and tormented history. Many thanks to the guys that, with time, contributed to the general public knowledge.

Pavel Korensky <>,
Andre M. Hedrick <>,
Christopher J. Reimer <>,
Kevin D. Smolkowski <>,
Werner Panocha <>,
Steven Freed,
Russell Kroll.

26 November 1999

additions by:

Kern Sibbald <>
21 December 1999

additional credits by:

Riccardo Facchetti

08 February 2000