Design Ideas: June 9, 1994

The 22V10 series of PLDs in 24DIP300 or 28PLCC packages can substitute both physically and functionally for the now-discontinued 74ALS844 and 74ALS880 latches. Beware, however, that some early PLCC versions of the 22V10 do not have compatible pinouts. Listings 1 and 2 contain the PLDASM source code for 22V10s that emulate the 74AS/ALS844 and 74AS/ALS880, respectively.

Determine whether your application can tolerate the performance differences between the PLDs and the AS/ALS originals. Pay particular attention to the I_OH source and I_OL sink capability your application requires. 22V10s source and sink only -3.2 and +16 mA compared with the ALS873/880's -2.6 and +24 mA and the AS873/880's -15 and +48 mA. Thus, these PLDs may not be a viable solution if your application drives a bus or a heavy load. Also, select the appropriate speed grade of the PLD, again depending on your application's requirements. You can download the ZIPfile which contains the writeup and listings— including simulation specifications. (DI #1430).