Such a slot was designed by ACR Special Interest Group in 2000 as a successor to AMR slot. ACR was designed to have backward-compartibility with AMR (not as incompartible Intel's CNR slot - another successor of AMR).
Correct designation is Advanced Communications Riser (also Advanced Communication Riser, note no ending s in a second word). Sometimes incorrectly referenced as Advanced Card Riser or Audio/Communications Riser.
This slot uses a slightly repositioned and reversed 5V PCI slot; initially it was designed to replace the last one ISA slot that remained on motherboards at a time. But while ISA slot was able to share place with nearest PCI slot (e.g. you may install either PCI card or ISA card using one placeholder of the chassis, as component side of ISA and PCI cards are different), ACR was not able (then, of course, ACR may share its place with ISA slot, but designs that implement it are unknown).
Generally these slots used card orientation as with AMR/PCI (NVIDIA nForce 420-D Reference Board, QDI KD7 (K7V 8366), Biostar M7VIB, Chaintech CT-7VJD2, 7NJS, CT-6VJD, Shuttle AV32, Transcend TS-APR3) to make use of AMR cards possible. Then B is the component (main) side of board, A is the solder side.
ASUS have been seen making this slot with CNR/ISA card orientation (ASUS A7N266, A7V266, CUV266); it allows this slot to share place with nearest PCI slot, but makes AMR and other ACR cards incompartible with it; ASUS produced its own ACR-like cards (ACR-A6CH). ASUS board's component side is A, and solder side is B.
VIA VT5431E Reference Board 2 appears to have slot positioned as AMR/PCI, but it appears slightly shifted to nearest PCI slot. It is unknown accurately, is this really true or not.
SiS 635 Reference Board (one of the earliest boards with ACR support) appears to have this slot shift much more to card bracket than required; it looks like a developer's mistake.
Generally has the same color as PCI (older slots) or specific blue (more recent ones), may be brown as AMR.
Pinout is as follows:
Light green (Rows 1-23 with keyway) = Current AMR Connections Blue (Row 24) = EEPROM Connections Cyan (Rows 25-35, 45-55) = MII/GPSI Connections, interfaces 1 and 2 Yellow (Rows 36-44) = DSL Connections Red (Rows 56-60) = Wireless Connections (Future implementations)
B1
Audio_Mute#
AUDIO_PWRDN
A1
B2
GND
MONO_PHONE
A2
B3
MONO_OUT/PC_Beep
RESERVED
A3
B4
A4
B5
A5
B6
Primary_DN#
A6
B7
.-12v
.+5v dual/+5v SB
A7
B8
USB_OC#
A8
B9
.+12v
A9
B10
USB+
A10
B11
.+5v Digital
USB-
A11
Keyway
B12
A12
B13
S/P-DIF-IN
A13
B14
A14
B15
.+3.3vDigital
.+3.3v dual/+3.3v SB
A15
B16
A16
B17
AC97_SDATA_OUT
AC97_SYNC
A17
B18
AC97_RESET#
A18
B19
AC97_SDATA_IN3
AC97_SDATA_IN1
A19
B20
A20
B21
AC97_SDATA_IN2
AC97_SDATA_IN0
A21
B22
A22
B23
AC97-MSTRCLK
AC97_BITCLK
A23
B24
EEPROMSCL
EEPROMSDA
A24
B25
TXD3
RXD3
A25
B26
MDIO
MDC
A26
B27
TXD2
RXD2
A27
B28
TX_ENABLE
RX_DV
A28
B29
TXD1
RXD1
A29
B30
DGND
A30
B31
TXD0
RXD0
A31
B32
TX_ERROR
RX_ERROR
A32
B33
TX_CLK
RX_CLK
A33
B34
A34
B35
COL
CRS
A35
B36
A36
B37
DSL_RDCLK
DSL_TDCLK
A37
B38
DSL_WAKE
DSL_IN:1
A38
B39
DSL_OUT:0
DSL_IN:0
A39
B40
A40
B41
DSL_OUT:1
VCXO_CTL
A41
B42
VAUX
DSLRST
A42
B43
TDFRAME
RDFRAME
A43
B44
A44
B45
A45
B46
A46
B47
A47
B48
A48
B49
A49
B50
A50
B51
A51
B52
A52
B53
A53
B54
A54
B55
A55
B56
Control1
Control2
A56
B57
RX_CLOCK
B.D. Data
A57
B58
A58
B59
S_DATA
S_CLOCK
A59
B60
RX CLOCK
A60
Pins are counted from card bracket to the deeper edge of board. As in AMR, keyway positions are not designated as pin positions, e.g. numbering is A1...A11, keyway, A12...A60.
Note the picture:
Slot name Card orientation
AMR Slot
+-A1--------A11-+-A12--------A23-+ |________________________
| [::::::::::::] [:::::::::::::] | | [] [] []
+-B1--------B11-+-B12--------B23-+ |
Standard ACR Slot
+-A1--------A11-+-A12------------------A60-+ |________________________________
| [::::::::::::] [:::::::::::::::::::::::] | | [] [] [] [] []
+-B1--------B11-+-B12------------------B60-+ |
ASUS ACR Slot
+-A1--------A11-+-A12------------------A62-+ |
+-B1--------B11-+-B12------------------B62-+ |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~PCI 5V Slot
+-A1-------------------A49-+-A52-------A62-+ |________________________________
| [:::::::::::::::::::::::] [::::::::::::] | | [] [] [] [] []
+-B1-------------------B49-+-B52-------B62-+ |
PCI 3.3V Slot
+-A1--------A11-+-A14------------------A62-+ |________________________________
+-B1--------B11-+-B14------------------B62-+ |