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IBM PC XT (5160) BIOS ROM Replacement

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IBM 5160 64-256KB system board BIOS ROMs 1501512 and 6359116
The 11/08/82 BIOS pair on a 64-256KB board: 6359116 (top) and 1501512 (bottom), the U19 and U18 chips of that revision. (Photo: Mister rf, CC BY-SA 4.0, cropped from Wikimedia Commons)

The BIOS of the IBM PC XT (5160) is held in two ROMs, in sockets U18 and U19 on the system board, together with Cassette BASIC. There were four revisions, known by date, and each chip carries a seven-digit IBM part number.[1] A 5160 BIOS cannot be flashed: replacing or upgrading it means fitting two new chips, normally EPROMs programmed with an image of the BIOS.[1] The sockets on early boards are wired for Mostek-type mask ROMs, and not every 27256 EPROM reads correctly in them.[2]

Symptoms

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  • "F6000 ROM" on screen, with the 08/16/82 or 11/08/82 BIOS: U19 is missing, faulty or badly seated. On those revisions U19 holds only part of Cassette BASIC, so the machine still boots and only Cassette BASIC is lost.[3][1]
  • "F8000 ROM", "FA000 ROM" or "FC000 ROM", with the same two revisions: part of U18 has become corrupt, or U18 is badly seated.[3]
  • A board that appears dead, with the 01/10/86 or 05/09/86 BIOS. Those revisions check U18 and U19 very early and halt the processor without displaying anything if either fails, and U19 holds part of the BIOS itself.[3][1] A faulty BIOS ROM is step 4 of minuszerodegrees' procedure for a dead 5150 or 5160 board.[4] The dead-board symptom has many other causes; start with the minimum diagnostic configuration.

Before replacing a chip that gives a ROM message, reseat it in its socket.[3]

Tools and parts

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  • An IC extractor.
  • A ROM image. Minuszerodegrees hosts images of all four revisions, each a pair for U18 and U19.[5]
  • Two 27256 or 27C256 EPROMs rated at 250 ns or faster, the figure in IBM's Technical Reference, or Winbond W27E257 EEPROMs.[1][5] On an early board, choose a make and model known to work in its sockets (see below).[2]
  • An EPROM programmer, or a programming service; minuszerodegrees lists some.[6]
  • A multimeter.[2]
IBM 5160 BIOS revisions
Date U18 / U19 (mask ROM) U18 / U19 (EPROM) Diskette support Notes
08/16/82 5000026 / 5000027 360K May not have been released publicly
11/08/82 1501512 / 5000027, or 1501512 / 6359116 360K; 720K partly Chip types vary
01/10/86 62X0851 / 62X0854 62X0852 / 62X0853 360K, 720K 27256
05/09/86 62X0890 / 68X4370 59X7268 / 62X0819 360K, 720K 27256

Source: minuszerodegrees.net.[1]

Minuszerodegrees considers an upgrade to the 05/09/86 revision valid on any 5160, with three caveats for both 1986 revisions: some third-party floppy controllers will read 1.44 MB diskettes but not boot from them, the IBM 5161 Expansion Unit may not be supported, and the 1.2 MB drive code needs more than a high-density controller to be used.[1]

Procedure

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Check the socket wiring

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Early 5160 boards wire U18 and U19 for Mostek MK37000 and MK38000 series ROMs; later boards wire them for 27256 EPROMs, with pin 1 connected to +5 V instead of to pin 27.[2]

  1. With the power off and the chips removed, measure between pins 1 and 27 of the U18 socket and of the U19 socket. A reading of 0 to about 2 Ω means the early MK37000/MK38000 wiring.[2]
  2. On an early board, the socket takes pin 1 of the chip low for the first half of its 32 KB and high for the second half. Many 27256 and 27C256 data sheets require pin 1 (Vpp) at +5 V for every read, and such a chip has only its second half read correctly on an early board.[2]
  3. On an early board, choose a chip from the table below.[2]
27256-class chips reported in early 5160 sockets
Maker Part Type Works on early wiring
AMD AM27C256-150 EPROM Yes
Atmel AT27C256R One-time-programmable EPROM Yes
Atmel AT28C256 EEPROM (not a 27C256 equivalent) Yes
Intel D27256-20 EPROM Yes
National Semiconductor NMC27C256Q EPROM Yes
NEC µPD27C256 EPROM No
Signetics S27C256-15 EPROM Yes
ST Microelectronics M27C256B-12 EPROM Yes
Texas Instruments TMS27C256 EPROM Conflicting reports: some worked, some behaved like the NEC part
Texas Instruments SMJ27C256 EPROM Yes
Winbond W27E257 EEPROM Yes

Source: minuszerodegrees.net, from reports by members of the Vintage Computer Federation forums. Minuszerodegrees warns that makers may have changed their designs, and that 27256 and 27C256 chips bought on eBay and elsewhere may be relabelled.[2]

Program the chips

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  1. Download the image pair for the chosen revision.[5]
  2. Program the U18 image into one chip and the U19 image into the other, verify both in the programmer, and label them U18 and U19.[5][1]

Fit the chips

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  1. Switch off and unplug the machine and open the system unit; see the maintenance guide.
  2. Note which way round the chips in U18 and U19 are fitted, then lever them out with the extractor. The general method is in IC Removal and Socketing.
  3. Fit the new U18 and U19 chips in their own sockets, the same way round.
  4. Check that jumper pad E1 is not bridged. Bridging E1 disables both ROM sockets completely, for use with a ROM card in the expansion bus.[7]

Testing

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  1. Switch on. IBM's normal sequence is a cursor after about four seconds, the memory size counting up in 64 KB steps in the top left corner, and one short beep when the POST completes.[8]
  2. With a 1982 BIOS, there should be no F6000, F8000, FA000 or FC000 ROM message.[3]
  3. If an early board stays dead with new 27256-class chips, suspect the socket wiring and the make and model of chip before anything else.[2]
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References

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  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Minuszerodegrees.net, "IBM 5160 - BIOS Revisions": U18 and U19, the four revisions with mask ROM and EPROM part numbers, diskette support, U19 on the 1982 and 1986 revisions, the 05/09/86 upgrade and its notes, no flashing, 27C256 substitution, and 250 ns.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 Minuszerodegrees.net, "IBM 5160 - IC Sockets U18 and U19": early and late socket wiring, the pin 1 to pin 27 continuity test, the pin 1 (Vpp) problem with 27256 and 27C256 chips, the table of make-models reported to work, and the warning about relabelled chips.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Minuszerodegrees.net, "IBM 5160 - POST - Some errors": the F6000, F8000, FA000 and FC000 ROM messages and their causes, and the early halt of the 1986 BIOS if U18 or U19 fails.
  4. ↑ Minuszerodegrees.net, "IBM 5150 and 5160 - Faulty motherboard symptom", step 4 "Faulty motherboard BIOS ROM?".
  5. ↑ 5.0 5.1 5.2 5.3 Minuszerodegrees.net, "Images of BIOS ROM's for IBM 5150/5155/5160/5162/5170", IBM 5160 section: the four image sets, 27256 or 27C256 at 250 ns or faster, and the W27E257.
  6. ↑ Minuszerodegrees.net, "Some EPROM Programming Services".
  7. ↑ Minuszerodegrees.net, "IBM 5160 - Motherboard Switch/Jumper Settings", jumper pad E1.
  8. ↑ IBM, Personal Computer Family Service Information Manual, SA38-0037-00 (January 1989), chapter 4, p. 4-3: the responses to watch for during the power-on self test. Hosted as File:IBM Personal Computer Family Service Information Manual SA38-0037-00.pdf.


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