TAN-011 Datasheet, PDF (1/8 Pages) Exar Corporation – A Comparison between Exar’s XR-88C681

TAN - 011
A Comparison between Exar’s XR-88C681 and Motorola’s MC2681 DUART Devices
A Comparision between Exar’s XR-88C681 and Motorola’s MC2681
DUART Devices.
Introduction
This Applications Note briefly summarizes the differences between the XR-88C681 and
the Motorola MC2681 DUART devices.
In general, these two device appear to be very compatible with each other. Their pin outs
for both the 44 pin PLCC and 40 pin DIP packages are exactly the same. There are,
however, some firmware (programming) differences between the two devices, that must
be addressed by the user, if alternate sourcing between these two devices is to be
achieved. Some of these differences between the XR-88C681 and the MC2681 are
presented below.
1. XR-88C681’s Z-Mode
The XR-88C681 allows the user to Command the DUART into the Z-Mode (sometimes
referred to as the “Zilog” Mode). This mode can be useful if the user wishes to daisy
chain a series of DUART’s together, as shown in Figure 1.
VCC
INTRN
CPU
VCC
INTRN
IEI IEO
INTRN
IEI IEO
INTRN
IEI IEO
INTRN
IEI IEO
IACKN
IACKN
HIGHEST
IACKN
IACKN
PRIORITY
IACKN
LOWEST
Figure 1, An Illustration of XR-88C681 devices operating in a Daisy Chain, when
commanded into the Z-Mode.
A more detailed discussion of the XR-88C681’s Z Mode operation can be found in the
Data Sheet. However, for the sake of this report, let’s just say that the Z-Mode is a
manner which allows several DUARTs to be configured in a Daisy Chain, as presented in