
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
0
0
0
0
0
0
Bit 6
0
0
0
0
1
1
Bit 5
0
0
1
1
0
0
Bit 4
0
1
0
1
0
1
XR-88C681
MC2681
Null Command
Reset MR Pointer
Reset Receiver
Reset Transmitter
Reset Error Status
Reset Channel’s Break
Change Interrupt
Start Break
Stop Break
Set Receiver BRG Select
Extend Bit
Clear Receiver BRG Select
Extend Bit
Set Transmitter BRG Select
Extend Bit
Clear Transmitter BRG
Select Extend Bit
Channel A: Set Standby
Mode*
Channel B: Reset IUS
Latch*
Channel A: Set Active
Mode*
Channel B: Set Z-Mode*
Reserved - do not invoke
during operation
Reserved - do not invoke
during operation
Null Command
Reset MR Pointer
Reset Receiver
Reset Transmitter
Reset Error Status
Reset Channel’s Break
Change Interrupt
Start Break
Stop Break
Null Command
0
0
1
1
1
0
1
1
0
0
1
0
1
0
0
1
Reset MR Pointer
1
0
1
0
Reset Receiver
1
0
1
1
Reset Transmitter
1
1
0
0
Reset Error Status
1
1
0
1
Reset Channel’s Break
Change Interrupt
1
1
1
0
Start Break
1
1
1
1
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.