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APC Measure-UPS II / AP9612 Environmental Management Card and Probe Exploration / Information

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Note: This page focuses primarily on the older AP9162 board and the AP9512 probes that go with it. I have added some information on the newer AP9335T and TH products further below.

APC's AP9612 SmartSlot accessory senses temperature and humidity through up to two attached probes, provides a low current regulated 12 volt output and four connections for dry contact switches or sensors that may be monitored by the card's logic. The temperature probes attach externally to the card and may be situated some distance away.

AP9612 Measure-UPS II Environmental Management Card and Accessories

The AP9612 does not operate by itself. Another SmartSlot device that can collect data gathered by the AP9612 must be present. AP9618 and AP9619 management cards utilize the same probes as the AP9612, although APC chose to omit the humidity sensor with the bundled probe for some curious reason. (Both the AP9618 and AP9619 do support the gathering and display of humidity data if you use an AP9512TH or AP9512THBLK probe.) There is some suggestion based on APC "Smart" protocol reverse engineering and documentation that the data collected by a Measure-UPS board may be available over the UPS serial port to any interested software.

Despite being shown in the illustration above, the actual card (at least my example of it) does not have the power LED or reset button. There simply isn't room on the rear panel for either. Nor does there appear to be any provision for such functionality on the board. (Zooming into APC's illustration from the AP9612 user manual suggests that perhaps the "power" opening is not for an LED, but rather a connection point for power input. Why this would be needed is a total mystery, since the SmartSlot and external enclosures all provide power to installed accessories.)

I suspect the AP9612 is primarily intended to operate with the AP9606 Web/SMNP SmartSlot accessory, which will report on its presence and any sensor data via the web interface. I don't yet know if the early AP9605 SMNP-only SmartSlot accessory supports data retrieval from the AP9612 or not. If it does, it is only through SMNP reporting as no mention of the environmental monitor or its contact monitoring capability are shown in the telnet administration interface. An AP9617 card I tested was not initially capable of communicating with the AP9612. A little bit of prodding from the web management interface ("reset only lost environmental communication alarms") did get it working. Another person reports that this combination allows the use of up to three temperature/humidity sensors (on an AP9618 or 9619 board).

An 87C52 microcontroller oversees the AP9612 while an ADC0834 8-bit A/D converter digitizes all temperature and humidity input values. The A/D converter may also handle all dry contact monitoring duties.

APC's documentation for the AP9612 suggests that only one temperature/humidity probe is supported by the device, despite the presence of two probe connection points. Two probes are in fact supported by the AP9612 and return separate data feeds.

Since it seems that APC has purged some manuals for older products from their support web site, here is the owner's manual for the AP9612.

AP9512T / AP9512TH Probe Pinout (Female Socket)

NOTE: The pinout below is correct, though I still have no idea of pin 4's exact function.

If you have better information, please get in touch.

The following pinout assumes one is looking into the female connector on the AP9612 board itself.

6 Pin Mini DIN Connector (image credit: Wikipedia)
Pin Number
Function, Voltage or Signal
+ 12 VDC, Regulated
Common for Temperature/Humidity (both sensors read very high values with this pin disconnected)
No Connection

An LM358 dual operational amplifier integrated circuit likely serves to boost signals from both the temperature and humidity sensors inside the probe, presumably to offset the effect of cable loss on the miniscule signals returned by each sensor. Hence the requirement for a power source leading to the probe. Temperature is sensed by an LM35 IC.

(Yes, older revisions of this page said it was a thermistor, and I thought it was. That's the hazard of doing work very late at night when one ought to be in bed. Jeff Pethybridge wrote in long ago with a correction.)

Humidity is sensed by a TDK CHS-UGR device. All temperature-only probes that I have so far seen include the empty spaces for humidity sensing capability as well and could probably be modified.


My card dates from early 2002 or so and came with firmware version 4Sx. A sticker on the microcontroller bears a copyright from the year 2000 and indicates a checksum value of 20AD. No upgrades are possible short of replacing the 87C52 microcontroller, as it is a one time programmable part.

Since writing this, I've found an older AP9612 card (made in the 36th week of 1997) having firmware revision 4Jx, checksum 0FD9 and a copyright of 1995. Despite this earlier card having its second environmental monitor port covered over and marked as "RESERVED FOR FUTURE USE", the second port works just fine.

Seen but not yet proven to exist (!!) is a "4Kx" firmware revision.

If you know of other firmware releases for the AP9612 or what each release changes, please contact me! I've not tried to read out the firmware from any of my AP9612 boards, nor have I tried to disassemble it.

Cable Extension

I've found that a male/female six pin mini-DIN extension cable (sold for extending the connection between a PS/2 keyboard and a computer) fifty feet in length works perfectly well. I don't know how much further than that you could go. APC/Schneider has their own take on this, and says that lengthening sensor probe cables is not officially supported. They go on to say that using up to two (!) 12-foot PS/2 extension cables was tested by their engineers and found to work.

For newer hardware like the AP9335/AP9335TH (not the focus of this page), an RJ45 8P8C female/female "joiner" (usually used to extend Ethernet patch cables) ought to work but is likely equally unsupported by APC.


One of my cards reports temperatures (and, as best I remember) humidity data readings that are about twenty degrees/percent too high. The probes work correctly with other AP9612 and AP9618/19 boards. I'm not sure what the cause is. I suspect a passive component has drifted out of acceptable tolerances.

Hardware Revisions

Of the two boards I have, serial numbers JA0217013591 and WA9736128411, the later board is marked as 640-0810D Revision 4 and bears a 1998 copyright. The earlier board has a 1995 copyright and is marked as 640-0810B Revision 2. Both boards appear identical apart from different passive component values in certain positions.

Newer Hardware

APC's newer AP9631 and AP9635 UPS management cards utilize a new method of connection known as "universal I/O" to provide for the attachment of external devices. The same applies to newer APC PDUs, which are based on slightly cut down versions of the UPS management cards. This expansion platform is also present on some Netbotz devices, about which I know absolutely nothing.

Although the AP9631 and AP9635 are both capable of utilizing a sensor that can detect both temperature and humidity, APC continues to ship them with a temperature-only sensor. (I don't know why they are repeating the same "mistake" made with the earlier AP9618 and AP9619 boards.)

The "universal I/O" platform consists of a temperature only sensor, temperature and humidity sensors, and a relay/dry contact module. As far as I'm aware, there is no follow-on product to the AP9612 board, and you definitely can't upgrade an AP9630 card to have the universal I/O inputs if you later decide that you want them. I'm willing to bet that only a few people ever upgraded their AP9617 boards, and APC quietly dropped the capability to save on manufacturing costs and warehousing expense. All "universal I/O" peripherals connect to their host with an RJ45 8P8C male connector.

The newer temperature probe is of much simpler design than the older AP9512T/TH probes. It uses a simple thermistor, for which a 10 K ohm type will seemingly work as an acceptable replacement. The original part is said to be a "2700k" type. Pins 1 and 3 of the 8P8C RJ45 connector are used to indicate what type of "universal I/O" device is attached (with a 400 ohm resistor indicating a temperature sensor), while pin 5 is connected to the shield, and pins 6 and 7 are connected to the thermistor. Pins 2, 4 and 8 are not connected in a temperature-only sensor. The unconnected pins likely do have some purpose for the temperature and humidity sensor, along with the relay/dry contact module.

Please bear in mind that the above information is not something I've personally verified. I have only the one AP9335TH probe, as well as one relay/dry contact module and I'm not terribly interested in descending upon either one with tools. The original source (which didn't work when I created this page) for this information is here. An archived link that is still functional is here.

To make a long story short, at least as far as temperature sensors are concerned, the differences between the new and old probes make the construction of a simple passive adapter impossible. You cannot directly use older probes with a newer management card, Netbotz device or PDU. Nor can you do the opposite.

APC "A-Link" Devices

I don't have any information at present about this interconnection system. I've only ever seen it on an older APC "environmental monitoring unit". If you do have some information about this, I'd be interested in hearing about it. It's seemingly used primarily (only?) for a remote display unit with temperature and humidity sensors. An A-Link "network" requires termination at each end. Although I have the terminators that go with the environmental monitoring unit, I haven't examined them in any detail.

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Copyright 2015-2019 by William R. Walsh. Some rights reserved. Your rights to reuse some or all of this material in other projects are fully detailed at the "terms and conditions" page available from the top level of this web server. Last updated 06/15/2019. Previously updated on 01/29/2017.

The AP9612TH card, manual and accessories image is copyright American Power Conversion Corporation and Schneider Electric. Use of this image and provision of the manual at no cost to rightful owners of the AP9612 hardware is believed to fall under the fair use doctrine within United States copyright law, where exceptions may be made for educational, news reporting, critical commentary and certain other uses that normally might constute infringement. I am not, however, a lawyer and this is not legal advice.

Mini-DIN port image sourced from Wikipedia, original here.
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