infrastructure is still powered up and functioning). The cards will also continue to work for a short while after the UPS has been switched off either manually or due to a low battery condition. Beginning with the 2009 release of the SMT models (and later SMX and SURTD models), the old RS-232 data interface (a serial port with DB-9 connector) has been replaced with a 10-pin RJ50 socket and an RJ50-to-DB-9 cable connecting to the protected computer's serial port (together with a standard USB interface). Also, SmartSlot interface has been electrically modified in a backward-incompatible way and appropriately keyed mechanically. As stated by APC, "the new connection arrangement denotes the new signalling systems." These new serial and SmartSlot interfaces use the new Microlink signalling protocol which, unlike the previous APC protocol UPSLINK, has not been publicly documented. However, using the new AP9620 interface card, users are able to add support for the previous signalling protocol to the newer SMT and SMX models. Addition of this card makes the current models backwards compatible with older software built for the older Smart-UPS series. , an additional option has been made available in form of new firmware for certain SMT and SURTD models, adding publicly documented Modbus signalling capability to the proprietary Microlink protocol, allowing third-party developers to support UPS signalling and control on the newer Smart-UPS series. SmartSlot card models In late 2009, with the release of the SMT and SMX product lines, SmartSlot was migrated to a new communications platform utilizing the APC Microlink protocol. Both electrical and mechanical (slot keying) properties of the new SmartSlot make older SmartSlot cards incompatible with current Smart-UPS models. For example, current () Smart-UPS model SMT1500 is compatible only with AP9613, AP9620, AP9630 and AP9631 SmartSlot cards. This new card/slot design is referred-to as "Network Management Card 2" or NMC2. However, most NMC2 cards (including AP9613, AP9630 and AP9631) are backward compatible with older Smart-UPS models. In 2019, APC released Network Management Card 3 or NMC3 (AP9640/AP9641), superseding NMC2 cards. NMC 3 offers enhanced security (Security chip onboard), faster communication (1000BASE/T) and increased space for data and event logs. See also Apcupsd Network UPS Tools (NUT) References External links Technical Comparison of On-line vs. Line-interactive UPS Designs APC White Paper #79, by Jeffrey Samstad and Michael Hoff Electric power systems components Uninterruptible power supply