Home

IBM 5170 Motherboard  -  ROM access time


ROM's that IBM used

•	Per here, some Intel D27128 EPROM's found on early type 1 motherboards (6 MHz) are 250 ns;  the data sheet (here) for the Intel D27128 (no suffix) indicates 250 ns.
•	Per here, some EPROM's found on 8 MHz motherboards are 170 ns;  the data sheet (here) for the Intel D27256-1 indicates 170 ns.

Information in technical references

•	The March 1984 edition of the technical reference for the IBM 5170, covers the type 1 motherboard (6 MHz).  In the ROM subsection of that technical reference is, "Its access time is 150 nanoseconds and ...".   IBM do not appear to be specifying a worse case access time; instead, simply indicating what was fitted.
•	The March 1986 edition of the technical reference for the IBM 5170, covers multiple types of motherboard.  In the ROM subsection is, "Its maximum access time is 260 nanoseconds (190ns) and ...".   There are two figures indicated, and the explanation for that appears earlier in the manual: "Where timing considerations between 6- and 8-MHz are different, the 8-MHz time is shown in parentheses."

Summary

Because of what we see in the March 1986 edition of the technical reference:

•  6 MHz motherboard:  Require ROM's of a rated access time of 260 ns, or faster.   ('Faster' is a lower ns figure, e.g. 200 ns)
•  8 MHz motherboard:  Require ROM's of a rated access time of 190 ns, or faster.   ('Faster' is a lower ns figure, e.g. 150 ns)

Note that the rated (maximum) access time is akin to the speed rating on car tyres; it is effectively a maximum speed that the chip is to be run at.  The IBM 5170 motherboards always run the ROM's at a fixed speed, irrespective of what ROM you fit.  So, on IBM 5170 motherboards, there is no performance to be gained by using a 'faster' ROM than that specified by IBM for the motherboard.