XR16C850 Datasheet, PDF(15/55 Page) Exar Corporation – UART with 128-byte FIFO’s FIFO Counters and Half-duplex Control

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XR16C850 Datasheet, PDF (15/55 Pages) Exar Corporation – UART with 128-byte FIFO’s FIFO Counters and Half-duplex Control

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XR16C850

PC Mode Interface (available on 44/48/52 pin ver-

sions only)

The PC mode is selected by making the SEL pin a logic

0 (GND). When the PC mode is selected, the 850

eliminates the external address decode logic circuitry

that is required. The PC mode is accomplished by

decoding the PC ISA bus address bits, A3 through A9

inside the 850. These addresses select the standard

PC COM ports: COM-1 (3F8-3FF Hex), COM-2 (2F8-

2FF Hex), COM-3 (3E8-3EF Hex), and COM-4 (2E8-

2EF Hex). Three inputs (S1-S3) are generally exter-

nally jumpered (logic 1 or logic 0) for selecting the oper

ating port. The selection bits are also associated with

a given PC interrupt. Interrupts IRQA, IRQB, and IRQC

function as three state outputs. MCR bit-3 must be set

to a logic 1 to activate these interrupts. The mapping for

the COM port 1-4 and their associated interrupt selec-

tions, IRQx are listed in Table 2, below. Figure 5 shows

a typical connection to the PC ISA bus.

In addition to the COM port addresses, the 850 de-

codes two additional printer addresses. These address

decodes select LPT-1 (printer port-1, 378-37F Hex), or

LPT-2 (printer port-2, 278-27F Hex). These ports are

intended to be compatible with PC or PC compatible

computer printer ports.

AEN*

IOR*

IOW*

IRQ4

IRQ3

IRQn

RESET

HDR2X3

-AEN

SEL

-IOR

-IOW

INTA

INTB

INTC

RESET

Figure 5. PC MODE INTERFACE

Rev. 1.20

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