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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 ROM's (or equivalent).  In later versions, the sockets are wired for a 27256 EPROM.



Early 5160 motherboards

Early versions of the IBM 5160 motherboard are wired for Mostek MK37000 series and MK38000 series ROM's (or equivalent).   Photos of example sets are in links at here.

27256 substitution

On the early motherboard, a 27256 or 27C256 EPROM (or equivalent) can, in most cases, be substituted in place of MK37000/MK38000.  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 Type Works ? Info source     Comment
AMD AM27C256 EPROM Yes modem7 at the VCFF  Make-model verified by electronic signature (device ID).
Atmel AT27C256R PROM Yes Moogle! at the VCFF  OTP EPROM  (an EPROM without a window for erasing)
Atmel AT28C256 EEPROM Yes Malc at the VCFF  Not a 27256/27C256 equivalent - see here.
National Semiconductor NMC27C256Q EPROM Yes mrmanse at the VCFF  
NEC µPD27C256 EPROM No mrmanse at the VCFF  More info at here.   Note that NEC normally do not print the "µP" prefix on their chips.
ST Microelectronics M27C256B EPROM Yes modem7 at the VCFF  Make-model verified by electronic signature (device ID).
Texas Instruments TMS27C256 EPROM Yes Malc at the VCFF  
Texas Instruments SMJ27C256 EPROM Yes pcdata76 at the VCFF  
Winbond W27E257 EEPROM Yes modem7 at the VCFF  See here.


Note: For the IBM 5160, use ROM's that are rated at 250 ns, or faster.  (Reference.)

Note: Be aware of possible relabelling of 27256 EPROM's that you source from eBay.



Later 5160 motherboards

At some point, IBM switched to using 27256 EPROM's, 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.  A documentation oversight ?

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.