Ampro Corporation 486E manual ROM-BIOS Installation of Parallel Ports

Little Board™/486e Technical Manual

ROM-BIOS Installation of Parallel Ports

LPT1 is normally assigned to the primary parallel port by the BIOS, LPT2 to the secondary parallel port if present, and so on. However, the BIOS scans the standard addresses for parallel ports and assigns LPT designations in the order it finds them. Thus, a secondary parallel port (at address 278h) can be assigned LPT1 if there is no primary port.

Note

The scan order is 3BCh, 378h, 278h.

Standard and General Purpose I/O Operation

The parallel port can be used as a standard output-only printer port or as a general-purpose digital I/O data port (Table 2–17). The bi-directional mode can be very valuable in custom applications; it might be used to control one of the following:

!Parallel-connected external peripherals

!An LCD display

!Scan keyboards

!Sense switches

!Interface with optically isolated I/O modules

All data and interface control signals are TTL-compatible.

Table 2–17. Parallel Port Use

Signal Type	Number of Lines	Function	Output Drive
Data	8 lines	Write Only	24 mA @ .5V
			12 mA @ 2.4V
Control	4 lines	Read/Write	12 mA @.5V
			4.7K PU
Status	5 lines	Read Only	--

The Bi-directional control register can be directly accessed without using the BIOS. The base address is 37Ah when the port is configured as the primary parallel port, and 27Ah when the port is configured as the secondary parallel port. Changing bit 5 can dynamically change the port between input and output modes. A “1” in bit five sets the port to input; a “0” sets it to output. The following example code dynamically changes the primary parallel port’s direction. The code assumes that the port is in Extended Mode.

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