TAN - 011

A Comparison between Exar’s XR-88C681 and Motorola’s MC2681 DUART Devices

Table 1, Register Addressing of “Read Mode” Registers for the XR-88C681 and the

MC2681 devices

DUART Address Bits

DUART Device/Read Mode Registers

A3

A2

A1

A0

XR-88C681

MC2681

0

0

0

0

Mode Registers, Channel A

(MR1A, MR2A)

Mode Registers, Channel A

(MR1A, MR2A)

0

0

0

1

Status Register, Channel A,

(SRA)

Status Register, Channel A,

(SRA)

0

0

1

0

Masked Interrupt Status

Register (MISR)

Do not access

0

0

1

1

Receiver Holding Register,

Channel A (RHRA)

Receiver Holding Register,

Channel A (RHRA)

0

1

0

0

Input Port Change Register

(IPCR)

Input Port Change Register

(IPCR)

0

1

0

1

Interrupt Status Register

(ISR)

Interrupt Status Register

(ISR)

0

1

1

0

Counter/Timer Upper Byte

Register (CTUR)

Counter/Timer Upper Byte

Register (CTUR)

0

1

1

1

Counter/Timer Lower Byte

Register (CTLR)

Counter/Timer Lower Byte

Register (CTLR)

1

0

0

0

Mode Registers, Channel B

(MR1B, MR2B)

Mode Registers, Channel B

(MR1B, MR2B)

1

0

0

1

Status Register, Channel B

(SRB)

Status Register, Channel B

(SRB)

1

0

1

0

RESERVED

Do Not Access

1

0

1

1

Receiver Holding Register,

Channel B (RHRB)

Receiver Holding Register,

Channel B (RHRB)

1

1

0

0

Interrupt Vector Register

Do Not Access

1

1

0

1

Input Port

Input Port

1

1

1

0

Start Counter Command

Start Counter Command

1

1

1

1

Stop Counter Command

Stop Counter Command

Note: Differences between these two devices are in Bold-Italics

Shaded boxes denote “Address Triggered” Commands

Table 2 presents the DUART Address and the corresponding “Write” mode Register that

will be accessed for each of these two devices.