View Full Version : IBM PC XT (5160)

April 12th, 2011, 06:57 AM

The IBM PC/XT (Model 5160) was IBM's second computer in the PC series, following the success of the IBM PC (Model 5150).
The XT in IBM PC/XT stands for eXtended Technology and was introduced on March 8th, 1983.
The XT contained a few improvements over the 5150, including more expansion slots and native HDD support.
In 1984, the XT was overcut by the IBM AT (Model 5170), and in 1987 it was replaced by the 8086-based PS/2 models.
thumb|right|The IBM 5160
BIOS and Motherboard Revisions
200px|thumb|rigth|The first revision
There were two versions of the XT. The first version ("early model") supported 256kB of memory on the motherboard itself and was expandable up to 640kb of RAM with the addition of an 8-bit ISA expansion card. Introduced in 1986, the second version ("late model") included the full 640KB of RAM on the motherboard itself and a revised BIOS that cut the boot time in half while adding support for enhanced keyboards.
There is however no significant difference in the layout of the actual system board. The difference is how it was configured. Even the first XTs could easily be made into 256-640KB systems just by adding a jumper, a multiplexer at U84, and replacing the BIOS (for full support with all the features of the "later model" XTs). However, there were actually two or three revisions of the PCB layout. The first revision (Raw-PCB number 1501490) has U90 left empty labeled as "SPARE", it has TD2 installed, and it also has two smaller components (a resistor and a capacitor) located above the speaker connector. The second revision has some re-arrangements of lines around U90, has TD2 replaced with a resistor in most of the cases, and has the two components moved to around E4. This change was introduced when submodel 086 replaced the submodel 087 in the spring of 1984. A third revision may also have been made, but the only difference to the second revision seems to be in one of the address lines going to the ROM BIOS, allowing only 32KB chips (instead of both 8KB and 32KB chips) to be used. This revision may have taken place with the introduction of the "late model" XTs in 1986.
|+ Different submodels of the XT
! Submodel Nr. !! Production period !! General Informaton !! Motherboard !! Configuration
! 087
| From 08.03.1983 to 19.06.1984
| This submodel consisted of the standard XT setup. One Double-Density full heigth floppy disk drive with IBM-branded adapter, one 10MB full heigth fixed disk drive with Xebec-branded controller, 128KB of RAM, either MDA or CGA, and perhaps some additional IBM-branded cards. This was the only submodel to contain the first revision of the motherboard.
| Rev 0
| 64-256KB Early configuration
! 086
| From 19.06.1984 to 02.04.1987
| This is practically the same as submodel 087, but with only two small differences. Those differences are that it was shipped with twice the amount of RAM (256KB), and it got the new "Rev 1" motherboard (see pharagrap above this table).
| Rev 1, or modified Rev 0
| 64-256KB Early configuration
! 068/078
| From 02.04.1985 to 02.04.1987
| Those two submodels are almost the same. They were both shipped with 256KB of RAM and a MDA adapter for monochrome graphics, and they both lacks the fixed disk drive. The only difference between the two submodels is that submodel 078 got two Double-Density full heigth floppy disk drives, where submodel 068 only got one. Those submodels was intended to be low-cost options to people who only tended to use them as computerized typewriters.
| Rev 1
| 64-256KB Early configuration
! 088/089
| From 02.04.1986 to 30.06.1987
| Those submodels are the ones most regularely refered to as "Late model" XTs (In fact it's only a configuration, see the pharagraph above this table for more info). They are configured with 512KB of RAM, they came with a 20MB fixed disk drive, and an upgraded BIOS, however, the rest is basically the same as submodel 086. The difference between the two submodels is that submodel 089 came with the Extended model-M keyboard (the version without led indicators for caps/num/scroll-lock), while submodel 088 came with the regular 83-key keyboard.
| Rev 1, modified Rev 1 or Rev 2 (research needed, may have something to with address line A14 of BIOS ROM chips)
| 256-640KB Later configuration
! 267/277/268/278
| From 02.04.1986 to 30.06.1987
| Basically submodel 068 and 078 made into "Late model" XTs. The only difference is that they got an upgraded BIOS as they still only got 256KB of RAM (Migth be 1 bank of '41256's instead of four banks of '4164's). The differences between the submodels is that submodel 268 and 278 came with Model M keyboards while submodel 267 and 277 came with 83-key keyboards. submodel 268 and 267 emerged from submodel 068 and submodel 278 and 277 emerged from submodel 078.
| Rev 1, modified Rev 1 or Rev 2 (research needed, may have something to with address line A14 of BIOS ROM chips)
| 256-640KB Later configuration

Like the IBM PC (Model 5150) the XT came with a BASIC interpreter in ROM. The only ways to access this were:
Disconnecting the hard drive and leaving the floppy drive empty
Using the BASICA program from a floppy (which included the extensions for using the disk drives)
Using a debugger to manually invoke a BIOS call.

Specifications and Upgrades
The standard 1983 XT included:
An Intel 8088 running at 4.77MHz (with a socket for an 8087 math coprocessor). AMD 8088 CPU's were also used in some models.
128K of RAM, expandable to 640KB
Full height 5 1/4" double sided floppy drive capable of holding 360KB of data
10MB Seagate ST-412 hard drive
an Asynchronous Adapter (serial card)
130W Power Supply
Eight 8-bit expansion slots
PC-DOS Operating System 2.0
Monochrome MDA video adapter options

The later models had 256KB RAM, half height floppy drives, an option for a 20Mb hard drive and an 'enhanced' keyboard (similar to a Model M). In 1985 IBM introduced a lower cost unit that was fitted with two full height floppy drives and had no hard drive.
Some after market upgrades included:
CGA graphics card and monitor
EGA graphics card and monitor
256KB ISA ram expansion card

The 5160 today
In more recent years some enthusiasts have been able to add:
IDE hard drives and controllers (see XTIDE project)
ISA VGA adapters (see 8bit Friendly ISA VGA cards)
Ethernet LAN cards
1.2MB 5 1/4" floppy drives
1.44MB 3 1/2" floppy drives

Jumpers and adjustable components on the system board
Jumper Settings
There are a total of 5 sets of jumpers labeled E1-E5 on the system board, and one block of switches. In addition, there is an empty socket (U84, on 64-256KB boards) and a trimable capacitor.
E1 is handy if you have an Expansion card with a replacement BIOS in the F000:0000 segment
E2 is useful if you want to configure your 256 KB XT into a 640KB XT
E2 page 3, E3 and E4 are for compability with the Intel 2118, a seldom used 1x16Kb RAM chip
E5 is probably from IBM's initial tests

Note: On all motherboards some or all of these jumpers were not present, and in order to use them you would need to solder some pins to the motherboard. In addition, jumpers jumpered by default (except for E2) are hard-wiered, so they have to be disconnected before a jumper block is soldered on.
|+ Component/Jumper Information
! Component/Jumper !! Function
! E1
| Inserting a jumper at E1 will disable the BIOS /CE decoding logic and the BIOS will disappear from the memory map.
! E2
| This is a set of 2 jumpers, and is provided to enable the user to change between 4 pages of the address-decode ROM. The pages of the ROM are
Page 1 (no jumpers): 64-256Kb operation. 4164 ICs being used in bank 1, 2, 3 and 4 (Default for 64-256KB configuration)
Page 2 (jumper between 1-2): 256-640 KB operation. 41256 in bank 1 and 2, 4164s in bank 3 and 4 (Default for 256-640KB configuration)
Page 3 (jumper between 3-4): 64 KB operations. 4164 in bank 3, or Intel 2118 in bank 1, 2, 3 and 4 (but only if E3 and E4 has been ajusted accordingly)
Page 4 (Jumpers between 1-2 and 3-4): 256-512KB operation. 128Kb memory ICs in bank 1, 2, 3 and 4

! E3
| This set of 2 pairs of jumpers is provided to change the address decoding technique. By normal/Default operation (jumpers between 1-2 and 7-8 ), two of the outputs from the address decode ROM is provided to the bank selector circuits, but when E3 is altered (jumpers between 2-3 and 4-5), address lines A14 and A15 are tied directly to the bank selector circuits instead. This will result in an address decoding with 16Kb between each bank, hence only the first 16Kb of each bank is being used. Note that a third pin of the decode ROM is still defining the address space the system memory is appearing in, so the DIP switches should be set to 64KB (as this is the lowest possible option) or E2 should be altered to page 3. E4 must also be altered (see below). With RAM ICs in all 4 banks, this configuration should work.
! E4
| This set of 2 jumpers are provided to change the address demultiplexing mode. In normal/Default operation (jumper on 1-2), the actual address line is decoded as 'a0,a1,a2,a3,a4,a5,a6,a7-a14,a8,a9,a10,a11,a12,a13,a15' (The 64Kb RAM ICs have 8 address lines, and the address is read in two cycles, first the low-order byte of the address, then the high-order byte of the address). The fact that a14 is between a7 and a8 will appear transparent to the programmer because all address lines are passed through on their separate line. However, if you try to read the memory with a logic analyzer, the data will be encoded. When altering E4 (jumpered on 3-4), a14 is replaced with a7. Because of this, a7 will appear twice. If you now imagine the Intel 2118, a 16Kb DRAM IC which don't use -5v/12v/-12v and which have only 7 address lines, the decoded address will appear as 'a0,a1,a2,a3,a4,a5,a6-a7,a8,a9,a10,a11,a12,a13' and thus those chips will therefore work. If this jumper is altered, E3 and E2 must be altered accordingly (see above).
! E5
| This 3-pin jumper is normally (by Default) jumpered between 1-2. In this configuration, pin 2 is grounded, and the open-collector keyboard line is buffered with a +VDC signal generated by U85. If the jumper is placed between 2-3, pin 2 is connected directly to bit 2 of I/O port 61h. When the jumper is in this position, the keyboard data line can be used to drive a small electrical device such as a speaker. In addition, your XT migth boot in manufacturing burn-in testing mode.
! U84
| When this chip is missing, address lines a16 and a17 are not multiplexed to the memory ICs. Since those two address lines are required by 256Kb DRAMs, a 74LS158 have to be inserted in order to use page 2 and 4 of the address decode ROM (see E2).
! C1
| This capacitor is used to trim the system clock sligthly. The CGA's composite circuit divides the system clock to generate the NTSC color burst signal, and by trimming the system clock, the color will change according to the trimming of C1.

Switch Settings
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|+ Switch Block Information
! Sw1 !! Function
! 1
On = Continuous Looping Post
Off = Normal POST (Power on Self Test)

! 2
8087 Math Co-Processor interrupt line
On = Disabled
Off = Tied to the NMI Circuits (enabled)

! 4/3
Filled Motherboard Memory Banks
On/On = Bank 0 only
On/Off = Bank 0 and 1
Off/On = Banks 0, 1 and 2
Off/Off = All 4 banks

Exception: If E2 is set to page 3, or if the address decoding ROM has been replaced. In the first case; the operation is the same for all setings of Sw1 3 and 4. In the last case; refer to the documentation for the new decoder ROM for what Sw1 3 and 4 represent.
! 6/5
Video Adapter Type
On/On = Video Adapter with on board BIOS or no video adapter installed
On/Off = Color Graphics Adapter - 40x25
Off/On = Color Graphics Adapter - 80x25
Off/Off = Monochrome Display Adapter, or more than one display adapters whereas one of them is a Monochrome Display Adapter

! 8/7
No. of Floppy Drives
On/On = 1 Floppy Drives
On/Off = 2 Floppy Drives
Off/On = 3 Floppy Drives
Off/Off = 4 Floppy Drive

Trouble Shooting
=== Beep Codes ===
No beep - Power supply, system board problem, disconnected CPU, disconnected speaker, or grounded keyboard data-line.
Continuous beep - Power supply, system board, or keyboard problem.
Repeating short beeps - Power supply or system board problem or keyboard.
1 short beep - Normal POST - system is OK.
2 short beeps - POST error - error code shown on screen.
1 long, 1 short beep - System board problem.
1 long, 2 short beeps - Display adapter problem (MDA, CGA).
1 long, 3 short beeps - Enhanced Graphics Adapter (EGA).
3 long beeps - 3270 keyboard card.

Further Reading
IBM PC/XT Technical Referance Manual

External Links:
url]http://www.vintage-computer.com/vcforum/showthread.php?t=9693[/url] The Vintage Computer Forums thread on the IBM 5160 BIOS revisions]