TAN - 014

A Comparison between Exar’s XR-88C681 with Signetics’ SC26C92 DUART Devices

Table 2, Write Register Addressing

A3

A2

A1

A0

SC26C92

XR-88C681

0

0

0

0

Mode Register A (MR0A,

MR1A, MR2A)

Mode Register A (MR1A,

MR2A)

0

0

0

1

Clock Select Register A

(CSRA)

Clock Select Register A

(CSRA)

0

0

1

0

Command Register A (CRA)

Command Register A (CRA)

0

0

1

1

Tx Holding Register A

(THRA)

Tx Holding Register A

(THRA)

0

1

0

0

Aux. Control Register (ACR)

Aux. Control Register (ACR)

0

1

0

1

Interrupt Mask Register

(IMR)

Interrupt Mask Register

(IMR)

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 Register B (MR0B,

MR1B, MR2B)

Mode Register B (MR1B,

MR2B)

1

0

0

1

Clock Select Register B

(CSRB)

Clock Select Register B

(CSRB)

1

0

1

0

Command Register B

Command Register B (CRB)

1

0

1

1

Tx Holding Register B

(THRB)

Tx Holding Register B

(THRB)

1

1

0

0

Reserved

Interrupt Vector Register

(IVR)

1

1

0

1

Output Port Configuration

Register (OPCR)

Output Port Configuration

Register (OPCR)

1

1

1

0

Set Output Port Bits

Command

Set Output Port Bits

Commands

1

1

1

1

Reset Output Port Bits

Command

Reset Output Port Bits

Command

Note: Differences between the two devices are written in Bold Italics

Shaded boxes denote “Address Triggered” commands.