IBM 5160  -  IC Sockets U18 and U19

On the 5160 motherboard, there are two IC sockets that hold the BIOS ROM chips, sockets U18 and U19.

On early versions of the motherboard, the sockets are wired for Mostek MK37000 series and MK38000 series ROMs.  In later versions, the sockets are wired for a 27256 EPROM.

Early 5160 motherboards

Early versions of the IBM 5160 motherboard are fitted with Mostek MK37000 series and MK38000 series ROMs (U19 = MK37000, U18 = MK38000], or equivalent.  A photo of an example set is here.

27256 substitution

On the early motherboard, a 27256 (or 27C256) EPROM can, in most cases, be fitted in place of the MK37000 and MK38000.  But at least one make/model of 27256 (or 27C256) has been found to be unsuitable.  The following table shows what is known to work and what is known not to work.

Make Model Works ? Info source     Comment
ST Microelectronics M27C256B Yes modem7 at the VCF  Make/model verified by electronic signature (device ID).
AMD AM27C256 Yes modem7 at the VCF  Make/model verified by electronic signature (device ID).
National Semiconductor NMC27C256Q Yes mrmanse at the VCF  
Texas Instruments SMJ27C256 Yes pcdata76 at the VCF  
NEC µPD27C256 No mrmanse at the VCF  More info at here.   Note that NEC normally do not print the "µP" prefix on their chips.

Note: For the IBM 5160, use ROMs that are rated at 250 ns, or less.  (Reference.)

Note: Be aware of possible relabelling of 27256 EPROMs that you source from eBay.

Later 5160 motherboards

At some point, IBM switched from using Mostek MK37000/MK38000 series ROMs to using 27256 EPROMs, or equivalent.  (Examples.)

To accomodate that change, the wiring of sockets U18 and U19 was changed.  Pin 1 is no longer connected to pin 27; instead connected to the +5 volts line.

It is probable that the change was made in the move from the 64-256KB version of the motherboard to the 256-640KB one.  However, contradicting that, is that the circuit diagram of the 256-640KB motherboard in the MAR86 edition of the technical reference, still shows the earlier pin 1/27 wiring.

The photos at here show the rewiring of the U18/U19 sockets on a 256-640KB labelled motherboard that I have, this motherboard being one that I believe IBM upgraded from a 64-256KB one.  Another one is shown at here, excepting the mod is all done on the component side of the PCB.