See also the main Model M Page.
Content created by John Szybowski, Sydney, AU
Modified by Major Tom.
These old IBM PS/2 keyboards have been a long-time favorite of mine. I
really like the positive, mechanical feel of their keys and find that I make
fewer typing mistakes than with the cheap, plastic and rubber membrane ones
which are commonly available today.
The one which I am using was manufactured in 1990 (and (c)1984,
according to the label on its base) and is still reliable in daily use.
These keyboards are broadly known as the 'IBM Model M' and more
specifically the one which I am using is 'Part No 1391401'.
Its probably the only computer component from that era which is still
usable with PC systems today; long after the 16 MHz processors, 20 MB
hard disks and EGA monitors have been sent to landfill.
When I recently upgraded my motherboard to a new ASUS P4T-E, I was
disappointed to find that this motherboard was unable to detect my old
keyboard. So after using a $10 plastic keyboard for a few weeks (and
cursing it numerous times), I decided to investigate the IBM keyboard
The solution turned out to be far simpler than I had first expected.
PC keyboards use a bi-directional clocked serial interface to
communicate with the motherboard. There are four conductors used (+5 V,
Ground, Clock and Data). The Clock and Data lines are open-collector
logic with pull-up resistors so that either end of the circuit can pull
the lines low. (See links below for further information).
After looking at the clock and data lines with a scope and comparing to
a newer keyboard, it seemed that the 'logic high' state on both Clock
and Data lines was fractionally lower when the IBM keyboard was
Another relevant factor is that the older technology used on the IBM
keyboard's controller PCB requires more power to operate than newer
keyboards. The IBM draws around 112 mA from the interface, whilst a
modern keyboard draws 1.2 mA. These figures are with the 3 status LEDs
(NumLock, CapsLock, ScrollLock) off. Each of these draw around 12mA
when lit on both keyboards.
The problem was cured by installing two 4.7k pull-up resistors (1/4 W,
5%); one each on the Clock and Data lines. The resistors were installed
directly onto the keyboard's controller PCB. The modification was
easier than expected as there were three convenient 'vias' (plated
holes through the board) which were on the correct signal traces and
could be used to mount the resistors.
Figure 1: IBM PS/2 Keyboard controller PCB (Modified)
Figure 2: Controller PCB (modified) close-up view
As there was only one hole available for the +5 V
connection to both resistors (just below pin 1 of the resistor network
RN2), the first resistor was inserted through the holes, with the
right-hand end of the second resistor being trimmed and soldered
directly to the lead of the first one.
My modified keyboard has been working well with the P4T-E motherboard
to this day.
I have also seen slightly different symptoms of incompatibility when
used with a particular Toshiba laptop. The keyboard works initially,
but after some time (minutes or hours) it will lock up, or appear to
have a stuck key (Ctrl in my case). This modification also appears to
fix these problems. Additionally, the same symptoms have also been
reported when using the keyboard with a Belkin KVM switch (and others).
Once again, the modification produced a satisfactory result.
If you have a PS/2 keyboard which needs this modification to
work with a new motherboard, then please send me an e-mail and tell me
the make/model of the motherboard. I will include it in my compatibility
table on this site.
I have also had a report that another keyboard has exhibited
the same behavior as the trusty old IBMs. It is the Northgate Omnikey
Ultra keyboard, and appears to be from around the same period (Mid
1980's). I haven't seen one of these myself, but it seems that they use
the same 'mechanical keyswitch' technology as the IBM. If you have one
of these and you're having problems, then the 'Interface Cable'
solution below may help.
Other Versions of the IBM Keyboard
Note: I have seen some keyboards of this type which have a different
PCB. These can be distinguished by the connection between the PCB and
the 3 LEDs (Num, Caps and Scroll lock). On the PCB described above, the
connection consists of flexible PCB traces the same as for the key
matrix. On the alternate PCBs, the connections are made using wires
with headers at either end. A similar modification may be performed to
the alternate PCBs, but you will have to identify the +5 V, Clock and
Data lines as they are located differently. A reader has kindly send me
two photos of his modification to one of these alternate PCBs.
Figures 3a, 3b: Alternate controller PCB
An Alternative - Make an Adapter Cable
There is another solution to the problem if you have a different type
of keyboard to the one which I modified, or if you don't wish to tamper
with its internal workings. It involves making an adapter cable which
plugs between the keyboard and the motherboard's keyboard connector.
The adapter cable contains the same modification (electrically) as the
one which I applied to the keyboard.
To make the adapter cable, it is best to buy a
straight keyboard extension cable - the type with a male plug at one
end and a female socket at the other. This cable can be cut in half and
the wires bared so that the two 4.7k resistors can be connected
appropriately. (And these cables are much cheaper and easier than
buying the individual plugs and wire).
All wires should be re-connected
straight-through and then the two resistors can be added. One resistor
connects between the +5 V and Data wires, the other goes between the +5 V
and Clock wires. Take care not to short any of the wires together and
finally ensure that everything is well insulated with heat shrink
tubing or electrical tape.
** NEW ** One of our readers, Ron Bean, has
designed and built a small adapter box. It functions in the same way as
the adapter cable, and is easier to put together. It is connected to
the PC with a male-male PS/2 cable (like the ones used to connect
most KVMs to a PC). He has kindly documented the entire process
(with pics). I have posted his
description here so that anyone wishing to build one can do so. The
step-by-step procedure is easy to follow.
Another Alternative - Get a USB to PS/2 Adapter
A USB to PS/2 adapter can solve the problem by providing an
alternative electrical interface to the keyboard, in place of the usual
one on the motherboard. However, as the adapter is now at the other end
of the keyboard cable, it can potentially have the same problems
communicating with the keyboard as many of the new motherboards.
In reports from readers, I have found that about half of these adapters
work well and the others don't at all. So if you're thinking of this
approach, check the table below to make sure that you get one that has
been used successfully by others.
Update Jan-Feb 2005: A reader has reported success in using this
USB to PS2 Adapter (pictured left, attached to the end of a Model M
cable). It is available from
www.clickykeyboards.com, for around USD$12. Their web site says
that they have performed a lot of testing with this unit and it has
worked in all cases.
I've recently acquired one of these adapters and put it through its
paces. I've tried it with my ASUS P4T-E, as well as a Toshiba Tecra
9000 and Satellite 4030 notebooks. It worked well in all cases. I was
also impressed by the fact that the adapter is directly supported by my
motherboard's BIOS, so it can be used to enter the BIOS settings
screens during the boot process (before any part of the operating
system has even loaded).
The USB to PS/2 adapter pictured at left has been found to work in most, but
not all, cases. It costs around USD$10. Some readers have told me that this
is sometimes branded as 'CP Technologies'. I have seen it on sale at a large
Australian electronics company (Jaycar) for AUD$19.95.
Update: I've had two e-mails from readers recently saying that the
adapter on the left didn't solve their problem, whilst others have had
success. I've also had other emails telling of mixed results with
various other adapters. Many of these adapters were unbranded, so the
only way to identify them would be with a picture.
To sum up, there many other USB to PS/2 adapters out there, some are
branded while others are generic. Some will work and some will not, so
its wise to ask if a refund (or exchange for another brand) will be
available if the adapter doesn't solve your problem.
Is your IBM keyboard missing its cable?
A few people have asked me for the wiring connections of the standard
IBM keyboard cable. This information wasn't easily found on the net, so
I took one of my cables and checked it with a multimeter. If you have a
keyboard but no cable, then don't throw it away! Look
here for some information on how to make
yourself a cable. I have also included the wiring details of a standard
IBM cable here for reference.
Which motherboards and laptops are affected?
(Limited list from my testing and reader's experiences)
If an item listed below has 'No' in the right-hand column,
then this means that the above modification is not required, as the
item should work directly with the Model M keyboard as-is.
|Type of Motherboard, Laptop|
or Other Keyboard Interface
ABIT BH6 v2
ABIT BX-6 r2
ASUS A7N8X Deluxe
Dell Dimension 8300
Gigabyte GA-K8N Pro
QDI Kudoz 7
Trigem Cognac (Socket 370)
Compaq Presario 1500TC
Toshiba Tecra 9000
Toshiba Satellite 4030
Toshiba Satellite Pro 405CS
KVM Switches (Note 3)
addLogix PowerReach KVM-401FMA
Belkin KVM Switch
Belkin F1D066 PS/2 KVM
Belkin F1DK102P KVM
IOGEAR GCS84A KVM
Linksys SVIEW04 V.2 KVM
PS/2 to USB Adapters, etc (Note 3)
Belkin PS/2 to USB adapter
Belkin F5U119-E USB to PS/2 adapter
'Cables to Go' PS/2 to USB adapter
Clickykeyboards.com USB adapter
CP Technologies USB adapter
GC (Great Quality) USB to PS/2 Adapter
IOGear GCS1734 USB KVM
IOGear GUC100KM PS/2 to USB Adapter
P.I. Engineering Y-Mouse USB (Hi-Power vers.)
QVS USB-PS2Y USB to PS/2 Key/Mouse
No (Note 7)
No (Note 6)
Yes (Note 5,6)
Yes (Note 6)
Yes (Note 1)
No (Note 8)
No (Note 2)
This motherboard was found to work with the Model M keyboard (without
the modification) if the keyboard was plugged into the PS/2 mouse
socket, and the mouse was plugged into the PS/2 keyboard socket! Some
motherboards have both functions present on both PS/2 sockets. (Further
details to follow in a future update).
Reports of both working and not working. I suspect that it may have to
do with individual conditions, like cable length, below.
It has been reported that certain KVMs and USB to PS/2 adapters cause
some keys to behave differently on the connected keyboards than would
be expected from a directly connected keyboard.. The most common
difference is the way that held keys are treated, such as <Ctrl>,
<Shift> and <Alt>. Another difference is key auto-repeat.
One reported case cites that for multiple held keys, the first key has
auto-repeat but the second and subsequent keys don't as each one is
Didn't work with the Model M, even after the resistor modification was
One kind reader reported the following: "D865PERL motherboards with
BIOSes prior to P17-0078 (which actually shows up as ".0078.P17." in
the middle of the very long BIOS version displayed on the powerup
screen) require a BIOS update, or else the keyboard will not work in
the BIOS screens and some boot managers". Prior to the BIOS
upgrade, the modification was applied but had no effect. The keyboard
exhibited intermittent operation.
Several readers have reported unusual problems with Intel motherboards.
One example is that the keyboard doesn't work to access the BIOS, but
works fine once Windows XP initializes it. In most of these cases, the
resistor mod will not fix the problem. The first option is to try a
BIOS upgrade (if available), otherwise get a PS/2 to USB adapter like
the one I've tested above.
A reader reported that the Model M was not detected under Windows 2000
from cold power-on, but worked after a subsequent reset. The Model M
works fine with this same motherboard under Windows 98 and Linux. In
this case, the modification did not fix the problem.
Many readers have had success with this adapter, with just a few
This KVM only allows a USB keyboard to be connected. A reader used the
USB-PS/2 adapter from clickykeyboards (mentioned earlier in this
article) to connect a Model M to the KVM. The Model M would only work
once modified. This was quite a surprise to me, as USB is very well
defined with regard to protocol compatibility and current supplied to
peripherals. I suspect that this would have been the same with any
USB-PS/2 adapter, as the clickykeyboards one has been the most
successful used by readers to date. I have no other reports to confirm,
but perhaps the current supplied by the KVM is insufficient? If anyone
else has experience with this KVM then please let me know.
The modification described on this page will not affect the
behavior of the individual keys. It only modifies the electrical
signal levels on the interface and not the way in which the keystrokes
It seems that the length of the cable attached to the Model M keyboard
might have an effect on whether it works or not with selected
motherboards, laptops and KVMs. The Model M's came with two different
lengths of cable attached. In both versions, the coiled section was
about the same length, but the straight section between the PS/2
connector and the coiled part was different. On the short version, it
was around 55cm while the long version was about 195cm. It seems that
the short version cables are more likely to work with problematic
Here's a place to buy Model M keyboards, as well as USB adapters
An article on the IBM keyboards mentioned on this page
PC Keyboard FAQ
Help with IBM-PC keyboard interfacing
The IBM Model M Keyboard
Another site devoted to Model M Keyboards
A site on keyboard repairing & modding
Another company that's selling some used Model M Keyboards
Model M has its own node on everything
http://everything2.com/index.pl?node=IBM Model M Keyboard
Belarc Advisor: Helps you to determine your motherboard type
The modifications described here are a record of those
which I performed on my keyboard. If you choose to modify your keyboard
in a similar manner, then you accept all responsibility for any
possible damage to your keyboard and/or other computer system
IBM is a registered trademark of International Business Machines.