AOpen, a subsidiary of Acer, was a  manufacturer of motherboards and sound cards for the IBM PC and its compatibles during the latter few years of the DOS era, first founded in late 1996.
        Based in Taipei, Taiwan, they started as the "Open System Business Unit" of Acer Computer Inc. Today they specialise in ultra small form factor platforms. Their website can be found at https://www.aopen.com/.
        
          
          | AP53Year: 1996Type: Socket 7
 Form Factor: Baby AT
 Power Connector: AT
 Chipset: Intel 430HX
 Expansion: 3 x 16-bit ISA, 4 x PCI
 Memory: 4 x 72-pin SIMM (ECC and non-ECC).  Max 512 MB.
 BIOS: AMI PnP
 BIOS Dates: 06/13/96
 Known Board Revisions: 3.0
 This board supports Intel Pentium from 75 MHz up to 200 MHz (3.3V CPUs), as well as Cyrix 6x86 P120+ up to P166+ CPUs. Supports 66 MHz FSB.         Comes with 512 KB of L2 cache onboard.  User Manual | 
          
          | AP55CSYear: 1996Type: Socket 7
 Form Factor: Baby AT
 Power Connector: AT
 Chipset: SiS 5501/5502
 Expansion: 4 x 16-bit ISA, 4 x PCI
 Memory: 4 x 72-pin SIMM (ECC and non-ECC).  Max 128 MB.
 BIOS: Award PnP
 BIOS Dates:
 Known Board Revisions:
 Supports AMD K5 and K6, Cyrix M2, and Intel Pentium.66 MHz FSB.
 256 KB L2 cache onboard.
 Single USB 1.1 port.
 User Manual         | 
          
          | AP57 Year: 1996Type: Socket 7
 Form Factor: Baby AT
 Power Connector: AT
 Chipset: SiS 5571
 Expansion: 4 x 16-bit ISA, 4 x PCI
 Memory: 4 x 72-pin SIMM
 BIOS: Award PnP
 This board supports Pentium P54C, Pentium MMX 150-233 MHz, Cyrix 6x86 P120+ up to P166+, AMD K5 from PR90 up to PR166, and K6 PR166/PR200. CPU core voltage ranges from 2.5V - 3.45V.FSB speed is 66 MHz only.
 Comes with 512 KB of L2 cache onboard.
 A single USB 1.1 port is available.
 For Cyrix chips, a jumper on the board (JP22) can be used to set Linear Burst Mode on.         User Manual         | 
          
          | AP5SYear: 1996Type: Socket 7
 Form Factor: Baby AT
 Chipset: SiS 5511
 Expansion: 4 x 16-bit ISA, 4 x PCI
 Memory: 4 x 72-pin SIMM. Max 256 MB.
 BIOS: Award PnP
 BIOS Dates: Rev 1.2
 Known Board Revisions: 2.0
 This board supports Cyrix 6x86 P120+ up to P166+ CPUs, AMD K5 and K6, and Cyrix M2.FSB speed is 66 MHz only.
 Comes with 256 KB of L2 cache onboard.
 A single USB 1.1 port is available.
 User Manual         | 
          
          |  AP5T
Year: 1997Type: Socket 7
 Form Factor: ATX
 Chipset: Intel 430TX
 Slots: ISA (3 x 16-bit), PCI (4)
 Memory: 4 x SIMM, 2 x DIMM
 BIOS: Award
 Known Revisions: 3.1
 This board comes with 512 KB   L2 cache onboard. This 430TX chipset board features CPU temp monitoring, 512 KB L2 Cache,   2x DIMM, 4x PS/2 sockets, 3x ISA / 4x PCI slots and outstanding quality.   This boards runs stable up to 83 MHz, but strangely I had to switch the   K6-233 down to 2.9 Volts (!) to run reliably at 250 MHz with the   Toshiba SDRAM. The Shuttle HOT-565 has similar features but no switching   voltage regulator. AOpen seems to become a reliable manufacturer for 83   MHz boards just the way ABit is doing with the AB-TX5. Both boards   support Ultra DMA/2, 64 MB cacheable area (SDRAM or EDO), no ECC   support. If you want to use the 233 MHz K6 processor, be aware you get   rev. 3 or newer, because you will need 3.2 V support for this CPU.   Revision 3.1 also has jumper settings for 2.2 Volts, which the K6 266   most likely will tolerate. The performance of the AOpen TX chipset   boards is fine once again, particularly with the K6. Core voltage support: 3.2V, 2.9V, 2.8V, and 2.2V. | 
          
          |  AP5VM
Year: 1997Type: Socket 7
 Form Factor: Baby AT
 Chipset: Intel 430VX Triton II/III
 Slots: 3 x ISA (16-bit), 4 x PCI.
 Memory: 4 x SIMM (2 banks), 2 x DIMM,
 BIOS: AMI WinBIOS
 CPU clock speeds from 75 - 200 MHz, bus speeds of 50 or 66 MHz. User ManualUser Manual (Revisions 2xx and 3xx)
 | 
          
          | AX5TYear: 1997Type: Socket 7
 Form Factor: ATX
 Chipset: Intel 430TX
 Slots: ISA (4 x 16-bit), PCI (4)
 Memory: 4 x SIMM, 2 x DIMM
 BIOS: Award
 Known Revisions: 3.0
 Revision 3 has some voltage selection jumpers that are probably reserved   for 2.1 or 2.2 V core voltage. Thanks to the 430TX chipset it features   the same limitations (64 MB cacheable, no ECC) and advantages (UDMA/2,   SDRAM support). Its performance is good, as with the AP5T. Other   manufacturers should take a look at some of the AOpen features, e.g. the   admirable stability at 75 and 83 MHz bus speed; you can say it's more   stable than fast at 83 MHz. The only comparable boards are the ABit   AB-TX5 and partially the Shuttle HOT-565/569. Supported bus frequencies are 60-83 MHz. User Manual | 
          
          | AX6BYear: Spring 1998Type: Slot 1
 Chipset: Intel 440BX (Seattle)
 Form Factor: ATX
 Power Connector: ATX
 Slots: 3 x ISA (16-bit), 4 x PCI, 1 x AGP
 Memory: 3 x 168-pin DIMM (Max 768 MB). Supports PC-66 and PC-100.
 BIOS: Award
 A decent 440BX motherboard with no compatibility issues, can run almost any type of SDRAM, and supports FSB settings from 66 up to 133 MHz.   | 
          
          | AX6BCYear: 1998Type: Slot 1
 Form Factor: ATX
 Power Connector: ATX
 Memory: 3 x 168-pin DIMM (Max 768 MB).
 BIOS: Award
 Supports the following CPUs: Intel Pentium II 233 to 400 MHz, Pentium III 450 to 700 MHz, and Intel Celeron 300A, 366, and 400. The memory slots support SDRAM and buffered SDRAM modules. User Manual         | 
          
          | AX37 Plus / AX37 ProYear: 199?Type:
 Form Factor:
 Chipset:
 Slots:
 Memory:
 BIOS:
 User Manual | 
          
            | AX59 ProYear: 1998Type:Socket 7
 Form Factor: ATX
 Chipset: VIA MVP3
 Slots: 2 x ISA (16-bit), 4 x PCI, 1 x AGP 2x
 Memory: 2 x 72-pin SIMM (FPM or EDO), 3 x 168-pin DIMM (PC-66 or PC-100 SDRAM)
 BIOS:
 CPU voltages available are 1.0V, 2.0V, 2.1V, 2.2V, 2.5V, 2.8V, 2.9V, 3.2V, 3.45V, and 3.52V.L2 cache can be either 512 KB or 1 MB, which provides 128 MB or 256 MB of cacheable memory respectively.
 BIOS: R2.35 (12 Jul 2000) - Adds support for AMD K6-2+ and K6-III+ CPUs. | 
          
          | MX59 Pro (II)Year: ?Type: Socket 7
 Form Factor: ?
 BIOS: R1.27 (26 Jul 2000) - Adds support for AMD K6-2+ and K6-III+ CPUs. |