TAN - 011

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

Table 5, Bit Formats for the Miscellaneous Commands (CRn[7:4]) in the Command

Registers for both the XR-88C681 and the MC2681 Devices

Bit 7

Bit 6

Bit 5

Bit 4

XR-88C681

MC2681

0

0

0

0

Null Command

Null Command

0

0

0

1

Reset MR Pointer

Reset MR Pointer

0

0

1

0

Reset Receiver

Reset Receiver

0

0

1

1

Reset Transmitter

Reset Transmitter

0

1

0

0

Reset Error Status

Reset Error Status

0

1

0

1

Reset Channel’s Break

Change Interrupt

Reset Channel’s Break

Change Interrupt

0

1

1

0

Start Break

Start Break

0

1

1

1

Stop Break

Stop Break

1

0

0

0

Set Receiver BRG Select

Extend Bit

Null Command

1

0

0

1

Clear Receiver BRG Select

Extend Bit

Reset MR Pointer

1

0

1

0

Set Transmitter BRG Select

Extend Bit

Reset Receiver

1

0

1

1

Clear Transmitter BRG

Select Extend Bit

Reset Transmitter

1

1

0

0

Channel A: Set Standby

Mode*

Channel B: Reset IUS

Latch*

Reset Error Status

1

1

0

1

Channel A: Set Active

Mode*

Channel B: Set Z-Mode*

Reset Channel’s Break

Change Interrupt

1

1

1

0

Reserved - do not invoke

during operation

Start Break

1

1

1

1

Reserved - do not invoke

during operation

Stop Break

* This is a chip level command (e.g., effects the operation of both channels). Hence this

command is invoked only if written to the specified Channel Command Register.