Specifications

Software Configuration
3–21
handshaking signals for increased throughput. Data flow is monitored by a watchdog timer (separate
from the board’s watchdog timer) to ensure reliable transfers.
 Extended Capabilities Parallel Port (ECP)—Compliant with the IEEE-1284 Extended Capabilities
Port Protocol and ISA Standard (Rev 1.09, January 7, 1993), developed by Microsoft. The ECP mode
provides the highest level throughput for the parallel port. It provides interlocking handshaking, a
16-byte FIFO buffer, DMA transfers (optional), hardware RLE data compression (optional), and
well-defined software protocols.
The low-level software interface to the parallel port consists of eight addressable registers. The address
map of these registers is shown in Table 3–6.
Table 3–6 Parallel Port Register Map
Register Name Address
Data Port Base address
Status Port Base address + 1
Control Port Base address + 2
EPP Address Port Base address + 3
EPP Data Port 0 Base address + 4
EPP Data Port 1 Base address + 5
EPP Data Port 2 Base address + 6
EPP Data Port 3 Base address + 7
Note: EPP registers are only accessible when in
EPP mode
3.11.1 Standard and Bi-Directional Operation
You can use the parallel port as a standard output-only printer port or as a bi-directional data port with up
to 12 output lines and 17 input lines. The bi-directional mode can be very valuable in custom
applications. For example, you might use it to control an LCD display, scan keyboards, sense switches, or
interface with optically isolated I/O modules. All data and interface control signals are TTL-compatible.
The default mode of the port is output only. To use the port as a bi-directional data port you must put it in
bi-directional mode with a BIOS call. For example:
;----------------------------------------------------------
; Code to set the parallel port mode
;----------------------------------------------------------
MOV AH,0CDh ; AMPRO command
MOV AL,0Ch ; AMPRO function
MOV BX,01h ; Extended mode (00 for output-only mode)
INT 13h
This code leaves the port in input mode. Once the port is in bi-directional mode, you can directly access
the control register without using the BIOS. The port address is 37Ah for LPT1 and 27Ah for LPT2. You