View source for OnSpec 90c26

= Introduction =

The OnSpec 90c26 was used on Adara and Microtek flatbed scanners in the
late 1990s as a parallel port to SCSI adapter.

= IBM-style Parallel Port Information =

==Port Registers==

 Port            R/W IOAddr      Bits    Function
 ----------          ------      -----   ----------------
 Data Out        W   Base+0      D0-D7   8 LS TTL outputs
 Status In       R   Base+1      S3-S7   5 LS TTL inputs
 Control Out     W   Base+2      C0-C3   4 TTL Open Collector outputs
     "           "     "         C4      internal, IRQ enable
     "           "     "         C5      internal, Tristate data [PS/2]
 
 Data Feedback   R   Base+0      D0-D7   matches Data Out
 Control Feedbk  R   Base+2      C0-C3   matches Control Out
     "           "     "         C4      internal, IRQ enable readback

The Feedback registers are for diagnostic purposes (except in bidirectional
ports, where Data Feedback is used for data input; and the IRQ enable C4).

==Pin signals and register bits==

 <= in   DB25    Cent    Name of         Reg
 => out  pin     pin     Signal          Bit     Function Notes
 ------  ----    ----    --------        ---     -----------------------------
 =>       1       1      -Strobe         C0-     Set Low pulse >0.5 us to send
 =>       2       2      Data 0          D0      Set to least significant data
 =>       3       3      Data 1          D1      ...
 =>       4       4      Data 2          D2      ...
 =>       5       5      Data 3          D3      ...
 =>       6       6      Data 4          D4      ...
 =>       7       7      Data 5          D5      ...
 =>       8       8      Data 6          D6      ...
 =>       9       9      Data 7          D7      Set to most significant data
 <=      10      10      -Ack            S6+ IRQ Low Pulse ~ 5 uS, after accept
 <=      11      11      +Busy           S7-     High for Busy/Offline/Error
 <=      12      12      +PaperEnd       S5+     High for out of paper
 <=      13      13      +SelectIn       S4+     High for printer selected
 =>      14      14      -AutoFd         C1-     Set Low to autofeed one line
 <=      15      32      -Error          S3+     Low for Error/Offline/PaperEnd
 =>      16      31      -Init           C2+     Set Low pulse > 50uS to init
 =>      17      36      -Select         C3-     Set Low to select printer
 ==      18-25   19-30,  Ground
                33,17,16

Note: Some cables, ports, or connectors may not connect all grounds.
Centronics pins 19-30 and 33 are "twisted pair return" grounds, while
17 is "chassis ground" and 16 is "logic ground".

"<= In" and "=> Out" are defined from the viewpoint of the PC, not the
printer.  The IRQ line (-Ack/S6+) is positive edge triggered, but only
enabled if C4 is 1.

Here's the same data grouped for ease of reference by Control Out and Status
In registers and pins.  (Data Out is straightforward in previous table).

 <= in   DB25    Cent    Name of         Reg
 => out  pin     pin     Signal          Bit     Function Notes
 ------  ----    ----    --------        ---     ------------------------------
 =>      17      36      -Select         C3-     Set Low to select printer
 =>      16      31      -Init           C2+     Set Low pulse > 50uS to init
 =>      14      14      -AutoFd         C1-     Set Low to autofeed one line
 =>       1       1      -Strobe         C0-     Set Low pulse > 0.5 us to send
 
 <=      11      11      +Busy           S7-     High for Busy/Offline/Error
 <=      10      10      -Ack            S6+ IRQ Low Pulse ~ 5 uS, after accept
 <=      12      12      +PaperEnd       S5+     High for out of paper
 <=      13      13      +SelectIn       S4+     High for printer selected
 <=      15      32      -Error          S3+     Low for Error/Offline/PaperEnd


= Physical Layer Protocol =

==PS/2 and Nibble Modes==

===Sending a Command===

{| border=1
| D0-D7 || <= Command Byte 
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x0D (lower #STROBE, #INIT, and #SELECT)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x0D (lower #STROBE, #INIT, and #SELECT)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x04 (lower #INIT)
|}

===Sending a Value===

{| border=1 
| D0-D7 || <= Value Byte
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x07 (lower #STROBE, #INIT, and #AUTOFD)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x04 (lower #INIT)
|}

===Read a Register (Nibble Mode)===

{|  border=1
| D0-D7 || <= Command Byte 
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x0D (lower #STROBE, #INIT, and #SELECT)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x0D (lower #STROBE, #INIT, and #SELECT)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x04 (lower #INIT)
|-
| C0-C3 || <= 0x06 (lower #AUTOFD and #INIT)
|-
| S3-S7 || >= Status Byte 1
|-
| C0-C3 || <= 0x04 (lower #INIT)
|-
| C0-C3 || <= 0x06 (lower #AUTOFD and #INIT)
|-
| S3-S7 || >= Status Byte 2
|-
| C0-C3 || <= 0x04 (lower #INIT)
|}

Read Value = (Status Byte 1 >> 4) &amp; 0xf) | (Status Byte 2 & 0xf0)

===Read a Register (PS/2 Mode)===

{| border=1
| D0-D7 || <= Command Byte 
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x0D (lower #STROBE, #INIT, and #SELECT)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x0D (lower #STROBE, #INIT, and #SELECT)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x04 (lower #INIT)
|-
| C0-C3, Tristate Enable || <= 0x26 (tristate data, #AUTOFD, and #INIT)
|-
| D0-D7 || >= Read Value
|-
| C0-C3 || <= 0x04 (lower #INIT)
|}

===Writing Register===

{|  border=1
| D0-D7 || <= Register Byte 
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x0D (lower #STROBE, #INIT, and #SELECT)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x0D (lower #STROBE, #INIT, and #SELECT)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x04 (lower #INIT)
|-
| D0-D7 || <= Value Byte
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x07 (lower #STROBE, #INIT, and #AUTOFD)
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| C0-C3 || <= 0x04 (lower #INIT)
|}

===Writing a Block===

{| border=1
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| D0-D7 || <= Byte 0
|-
| C0-C3 || <= 0x07 (lower #STROBE, #AUTOFD, and #INIT)
|-
| D0-D7 || <= Byte 1
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| ... 
|-
| D0-D7 || <= Byte N is even
|-
| C0-C3 || <= 0x07 (lower #STROBE, #AUTOFD, and #INIT)
|-
| D0-D7 || <= Byte N is odd
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
|}

===Reading a Block (Nibble)===

{| border=1
|-
| C0-C3 || <= 0x04 (lower #INIT)
|-
| C0-C3 || <= 0x06 (lower #AUTOFD and #INIT)
|-
| S3-S7 || >= Status Byte 0
|-
| C0-C3 || <= 0x04 (lower #INIT)
|-
| C0-C3 || <= 0x06 (lower #AUTOFD and #INIT)
|-
| S3-S7 || >= Status Byte 1
|-
| C0-C3 || <= 0x04 (lower #INIT)
|- 
| || Read Value = (Status Byte 1 >> 4) &amp; 0xf) &pipe; (Status Byte 2 &amp; 0xf0)
|-
| ...
|}

===Reading a Block (PS/2)===

{| border=1
|-
| C0-C3 || <= 0x24 (Tristate, lower #INIT)
|-
| D0-D7 || >= Read Byte 0
|-
| C0-C3 || <= 0x26 (tristate, lower #INIT and #AUTOFD)
|-
| D0-D7 || >= Read Byte 1
|-
| ...
|-
| C0-C3 || <= 0x24 (tristate, lower #INIT)
|-
| D0-D7 || >= Read Byte N even
|-
| C0-C3 || <= 0x26 (tristate, lower #INIT and #AUTOFD)
|-
| D0-D7 || >= Read Byte N odd
|}


==EPP Modes==

===Sending a Command===

{| border=1
| ADDR || <= Command Byte
|}

===Sending a Value===

{| border=1
| DATA || <= Value Byte
|}


===Writing a Register===

{| border=1
| ADDR || <= Register Byte
|-
| C0-C3 || <= 0x05 (lower #STROBE and #RESET)
|-
| DATA || <= Value Byte
|-
| C0-C3 || <= 0x04 (lower #RESET)
|}

=== Reading a Register ===

{| border=1
| ADDR || <= Register Byte
|-
| C0-C3, Tristate Enable || <= 0x24 (tristate data, lower #INIT)
|-
| DATA || >= Read Value
|-
| C0-C3 || <= 0x04 (lower #INIT)
|}

=== Writing a Block ===

{| border=1
|-
| C0-C3 || <= 0x05 (lower #STROBE and #INIT)
|-
| DATA || <= Data Byte 0
|-
| DATA || <= Data Byte 1
|-
...
| DATA || <= Data Byte N
|}

=== Reading a Block ===

{| border=1
|-
| C0-C3 Tristate || <= 0x24 (Tristate, lower #INIT)
|-
| DATA || => Data Byte 0
|-
| DATA || => Data Byte 1
|-
...
| DATA || => Data Byte N
|}


== Link Connection ==

=== Connect Link ===

{| border=1
| DATA || <= 0xFE
|-
| DATA || <= 0xAA
|-
| DATA || <= 0x55
|-
| DATA || <= 0x00
|-
| DATA || <= 0xFF
|-
| DATA || <= 0x87
|-
| DATA || <= 0x78
|-
| DATA || <= 0x20
|-
| C0-C3 || <= 0x05 (lower #INIT and #STROBE)
|-
| C0-C3 || <= 0x04 (lower #INIT)
|-
| DATA || <= 0xFF
|}

 Nibble_Command(2)
 Nibble_Value(0)
 Nibble_Command(2)
 NIbble_Value(IsNibbleMode ? 0 : 1)
 Write_Register(2, IsNibbleMode ? 0 : 1)

=== Disconnect Link ===

 Write_Register(3, 0)
 Write_Register(7, 0x48)
 Command(4)

{| border=1
| DATA || <= 0xFE
|-
| DATA || <= 0xAA
|-
| DATA || <= 0x55
|-
| DATA || <= 0x00
|-
| DATA || <= 0xFF
|-
| DATA || <= 0x87
|-
| DATA || <= 0x78
|-
| DATA || <= 0x30
|-
| C0-C3 || <= 0x05 (lower #INIT and #STROBE)
|-
| C0-C3 || <= 0x04 (lower #INIT)
|-
| DATA || <= 0xFF
|}
 
= OnSpec 90c26 Registers =

{| border=1
! Register !! Type !! Comments
| 0x00 || R/W || B0-B6: SCSI Target<br>B7: Initiator Mode
|-
| 0x01 || R/W || B0: SCSI Connected/Busy (R/O)
<br>B1: SCSI Enable
<br>B4: SCSI Bus State REQ (R/O)
<br>B5: SCSI Bus State MSG (R/O)
<br>B6: SCSI Bus State CD (R/O)
<br>B7: SCSI Bus State IO (R/O)
|-
| 0x02 || R/W || B0: Bi-Direction Mode Enable
<br>B5: Read SCSI Bus status into Register 0x01
<br>B7: SCSI Mode Enable
|-
| 0x03 || W ||
|-
| 0x04 || R || ASIC Version 
|-
| 0x07 || W || B3: Unknown<br>B6: Unknown
|-

= Link Layer Protocol =