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
| 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
| 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)
| 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) & 0xf) | (Status Byte 2 & 0xf0)
Read a Register (PS/2 Mode)
| 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
| 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) |
EPP Modes
Sending a Command
| ADDR | <= Command Byte |
Sending a Value
| DATA | <= Value Byte |
Writing a Register
| ADDR | <= Register Byte |
| C0-C3 | <= 0x05 (lower #STROBE and #RESET) |
| DATA | <= Value Byte |
| C0-C3 | <= 0x04 (lower #RESET) |
Reading a Register
| ADDR | <= Register Byte |
| C0-C3, Tristate Enable | <= 0x24 (tristate data, lower #INIT) |
| DATA | >= Read Value |
| C0-C3 | <= 0x04 (lower #INIT) |