8573 Common Devices

Locate Type-Model
Open the Case
How to fix a "Broken" Door
Plasma Display Information
   VGA, CGA and EGA resolutions
   Color graphics contrast
   Text mode contrast
   Auto-dim feature
   External PS/2 displays
   Color to Gray Mapping
   Turned-on Display
   Plasma Display Brightness
   Close Plasma Display
What is the Plasma Display?
Why does Plasma Display Get So Hot?
   "Cool" Windows Color Schemes
Plasma PCB
Remove Hard Drive
Miniture Right Angle KB Plug Identity
Minature KB Connector Pinout
External Storage Device Port
   External Storage Device Cable
     ESD Cable Pinout
     ESD Cable Hosiden Connector Pinout
      Cable Source
Remove power On Password

Locate the Type-Model 

Opening the Case

1. Strongly push below latch to release.
2. Work right to left. Lift edge of rear cover outward to clear catch.
3. After releasing all the five latches, lift and remove the rear cover. 

Note: There are two additional catches, one to each side of the carrying handle.When you lift up the rear cover, it pivots on them.

How to Fix a "Broken" Door
From Us, the god-Emperor of Microchannel
How many times have you seen the ports cover hanging loose off of a P70 or P75? Chances are, it ain't broke, just discomboobulated.

Open the case. (unscrew the three captive screws, pry out and up on the bottom edge). Pull up on the bottom edge and rotate it up on the pivots at the top rear of the case.

Look at the slots where the ports door hinges enter the case body. See the wide end? If you look on the interior of the case on the opposite of the wide ends, you will notice a pivot point for the metal hinge to fit into.

Remove the ports cover by pressing the hinge inwards (relative to the port cover).

Look at the "U" shaped arms of the hinge. Are they straight out from the metal strip? They should be at right angles to the metal strip. If they aren't, _carefully_ bend them so they are. 

Now reinsert the hinges through their slots and pop the hinge points back into the matching pivots. You may have to carefully press the hinges inwards, since the pivots have a lip around them. Shut door.

If the hinges want to pop free, carefully bend them outwards just a little more, so more force is exerted against the pivots,

Plasma Display Information
   The computer has a gas plasma display that shows program colors in up to 16-level monochrome shades with a VGA of 640 X 480 resolution. Some programs may not be designed to take full advantage of the plasma display, or could benefit from minor changes. Here are some tips for best results.

CGA and EGA resolutions-Programs using CGA and EGA resolutions are supported by this display; however, program screens will not use the entire plasma display area, and graphics shapes could be slightly distorted. How much area of the plasma display is used by VGA, CGA, and EGA is illustrated in the figure below. For best results, use the version of a program that supports VGA resolution.

Color graphics contrast-The plasma display automatically converts program colors into different monochrome shades. Some programs start with colors that lose contrast when converted. You can usually change colors from within the program to help increase the contrast.


A program will not start; parts of a program screen are missing-Some graphics programs, for example, Microsoft Windows i386 versions 2.1 and below, may not recognize the graphics capabilities of the plasma display and other PS/2 monochrome displays. This could appear as start-up difficulty, or faded/missing information. To correct this situation, put the plasma display (or the attached PS/2 monochrome display) in graphics mode before starting the program. To do this, type MODE CO80 at the DOS prompt, then press Enter.

Text mode contrast-The contrast in text (non-graphics) programs is best when the plasma display is in two-shade text mode. The display is in this mode when the computer is turned on, but could be put into graphics mode by a program. To return the display to text mode, type MODE MONO at the DOS prompt, then press Enter.

Auto-dim feature-The plasma display is automatically turned off to extend its life if no data is received from the system or entered from the keyboard or an auxiliary device for 10 minutes (default value). The default value can be changed by using the backup copy of the Reference Diskette. To change the default value, go to the Auto-Dim Time. The following shows the path to the Auto-Dim Time.

Main Menu>Set Configuration>Change Configuration>Auto-Dim Time

There are two ways to turn on the display again:
1. Pressing SHIFT
2. Moving the mouse.

If the mouse picks up vibration from a desk which turns on the display again, change the Auto-Dim Reset from Keyboard and Aux. Device to Keyboard Only. The following shows the path to the Auto-Dim Reset.

Main Menu>Set Configuration>Change Configuration>Auto-Dim Reset

External PS/2 displays-The computer supports all PS/2 displays with VGA resolution. If you use the same program with the plasma display or an external PS/2 display, the best choice for program colors could change depending on the display you are using.

Color to Gray Mapping-There are some applications that are developed for color displays but can be used on monochrome displays. When using such applications, reset the Color to Gray Mapping to Green Signal Only. Setting RGB Mixed Signals (default value) may result in an unpredictable display image. The following shows the path to the Color to Gray Mapping.

Main Menu>Set Configuration>Change Config>Color to Grey Remapping

Turned-on Display-The default value, Primary, turns on only one display that is considered by the system to be the primary display. When no external PS/2 display is connected to the system unit, the plasma display is considered the primary display and only VGA mode is supported. But when an external PS/2
display is connected, it is considered the primary display and VGA or XGA mode is supported for the external PS/2 display. The value, Plasma and External, turns on both the plasma display and an external PS/2 display. Only VGA mode is supported. The following shows the path to the Turned-on Display.

Main Menu>Set Configuration>Change Config>Turned-on Display

Plasma Display Brightness
   You can reverse the brightness of standard text and emphasized text in DOS through a simple command, BRT. This command lets you set standard text brighter than emphasized text, or vice versa. The Reference Diskette contains this command program and a program to install it. When you attach an external PS/2 display or use certain programs, the brightness is not selectable. That is, emphasized text is always displayed with brighter characters.

How to Install the Brightness Program
  To change the brightness levels, you must first install the brightness program from the Reference Diskette onto the fixed disk drive or bootable DOS diskette.
  1 Start computer using the operating system.
  2 insert the Reference Diskette .
  3 Type A:BRIGHT and press Enter.
  4 Follow the instructions on the screen.

After Installing the brightness program, use the following instructions on how to change the brightness level.

To Change the Brightness Level
Go to the DOS prompt (usually "A> " or "C> "), then:
   Standard text brighter than emphasized text, <BRT /H - Enter>
   Emphasized text brighter than standard text, <BRT - Enter>.
Note: The brightness program is used only in the default monochrome text mode of the plasma display. It is not used with graphic modes, color modes, or an external PS/2 display.

Close Plasma Display


First push back the lower edges into the case. Now push the upper edges back into the case. Seems odd, but it works.

What is the Plasma Display?
Peter Wendt writes:

Technically it is a sort of "cold cathode tube". It does not use a heater like radiotubes and the "cell" is filled with a neon-argon gas mixture. There is a negative cathode and a positive anode to which a "high voltage" (about 170V) is applied. The cathode is then covered with an electroluminence glow emitted by the excited electrons within the gas compound. The anode is a thin, semi-transparent metal coated sheet of low-temperature glass. We look through the anode into the grid of individual local gas cells. These orange "Plasma screens" are closer to the neon lamps that can be found in nightlights than to real tubes. They are grids of very tiny neon lamp cells on a common carrier and covering glass.

Another cousin within that technology is the "Nixie" display tube, where the multiple cathodes are formed like numbers, letters and signs. Or the Beckman / Sperry "flat panel displays" (early multi-segment displays) of the 70's. Modern plasma screens (as used in flatscreen TVs) are basically the same technology, but there is an additional layer with RGB triplets that light up when the underlying neon cell is activated. Brightness control is very tricky and mostly achieved by pulse-width modulating the corresponding cell. A cell can only be on or off, altering the applied voltage results in only marginal brightness change (below 10%). Therefore "new" plasma screens are an interesting mix of very old and very new technology. The basic technique goes back to the Geissler tubes of the 19th century, over the neon lamps of the late 19th and early 20th century and needs todays complex and fast computer / microcontroller support to work at all - generating the appropriate column / row / intensity control signals.

 Their advantage over TFT LCD is the far lower count of active elements - resulting in an immanently higher reliability and lower production losses as well as the absence of a "backlight". They are self-illuminating screens.

Why does the screen on the P7x get so hot?
Peter Wrote:
   Well - this is a symptom of its function. The orange screen is commonly called 'Plasma Display' (a hot word ;-) ) but the correct technical term is 'Gas Discharge Display'. The principle of function is in brief that you have a cell with a luminous gas between two electrodes for each dot (or pixel). The one electrode is Ground, to the other is a voltage applied between 70 and 100VDC. The gas in the cell is caused by this voltage to emit light. You know these lights on the front of your oven in the kitchen? same principle but much smaller and many of them combined in a large circuit.

   The controller circuits are soldered directly to the back of the screen. Both - the gas-discharge element and the controller circuits give up some heat which is neither dangerous nor unnormal. That's all. These screens are a high-voltage technology unlike to LCD-screens (these black on whites as used in most modern laptops) - these LCDs are low-voltage / indirect lighting technologies - however the lamp to backlight them is a high-voltage again. The plasma-display is a high voltage / direct lighting screen, which needs no backlight.

   The main disadvantage: some higher power-consumption, normally not possible to feed it from batteries, somewhat complicated shading / contrast control. Main advantage: rigid, temperature independent (while self-heating :-) ), not very much pressure sensitive. Makes a good hand-warmer in cold times .... !

"COOL" Windows Desktops
  WfW 3.1x - Control Panel>Color>Color Schemes, use "Plasma Power Saver". W95 - Display, set the desktop to black. Your display will stay slightly warm, but the display will be mostly black.

Plasma Control PCB M480L640HB53 (single row connector plasma screen)

CN1 Bottom grid controllers
CN2 Top grid controllers
CN3 Right grid controllers
CN4 Left grid controllers
F1 SOC 125mA
IC1 TI SN755787
IC2 MN5130
IC3 TI SN755783A
J1 Signal cable from plasma controller
J2 Power
J3 Brightness slider switch
X1 800NDK04 xtal
This PCB was mounted to a Matsushita Electric plasma display MD480L640PG4, P/N 64F8801

Hard Drive Removal

   Detach power cable from system board. 
   Detach SCSI cable from system board. 
   Remove both screws from the HD Bracket.
   Remove black screw from front corner foot.
   Pull drive cage up.

KB Plug Identity
  The P7x right angle connector is HRS # CL231-3004-2, part # 3130A-6PA Go to the Hirose Electric web page and look at their catalog HERE

T. Sunagawa says:
   Letter "A" after 3130 is missing. I did same mistake and asked for 3130-6PA thus I got a brusque reply "item not found".
   The letter following.3130 stands for the diameter of guide hole of each signal wire.
    A: guide hole 0.85 ( suitable for wire 0.75 to 0.83 )
 none: guide hole 1.00 ( suitable for 0.88 to 0.98 )

Keyboard Connector Pinout


27 Nov 01- Trying to convert between the four pin header and the common PS/2 port. My urge is to allow installing a short female PS/2 plug to stick an external PS/2 keyboard on. You might even want to create your own extension cable, because finding the special extension cable is about as likely as finding Elvis in your kitchen.

David Beem wrote:
    On second look it appears to be:

Pin 2 = +5VDC (Red on mainboard connector)
Pin 3 = Ground (Black on mainboard connector)
Pin 4 = Data (Green on mainboard connector)
Pin 5 = Clock (Yellow on mainboard connector)

    Still trying to get an "M" apart for cross-comparison.

External Storage Device Port
       The external storage device port is the equivalent of the B drive internal diskette port, as found in the PS/2 Model 70.  This permits attachment of externally powered devices like the IBM 4869 External Diskette Drive Model 001 and some other equipment manufacturers' backup devices.

PIN  I/O  Signal         PIN   I/O   Signal
 1   N/A  Ground           2    I    - Second Drive Installed
  N/A  Ground           4    O    - Reduced Write Current
 5   N/A  Ground           6   N/A     Reserved
 7   N/A  Ground           8    I    - Index
 9    O   - Motor On 1    10    O    - Drive Select 1
  N/A  Reserved        12   N/A     Reserved
13   N/A  Ground          14    O    - Direction
15   N/A  Ground          16    O    - Step
17   N/A  Ground          18    O    - Write Data
19   N/A  Ground          20    O    - Write Gate
21   N/A  Ground          22    I    - Track 00
23   N/A  Ground          24    I    - Write Protect
25   N/A  Ground          26    I    - Read Data
27   N/A  Ground          28    O    - Side 1 Select
29   N/A  Ground          30    I    - Diskette Change

Note: NC means No Connector, there is nothing in the socket. 

External Storage Device Cable P/N 23F2716 / FRU 23F3230
US4854895 A Shielded connector socket for connection with a multipin connector plug
  I think this is akin to the HDI-30 SCSI port on Apples.

     This cable attaches appropriate external backup devices and diskette drives.  It features a Hosiden HDI-30 connector that attaches to the external storage device port on a P70 / P75, and at the other end is an industry standard 37 pin D Shell connector to connect to an externally powered device.  The length of the cable is 35.5cm (14 inches).  This 30 pin port is the equivalent of the B drive internal diskette port (like in the PS/2 Model 70) and is reported as D:.

On the left is the DB37 connector that mates with the 4869 signal cable. On the right is the Hosiden HDI-30 connector that plugs directly into the P70/P75 external storage device port.

This side shot of the HDI-30 plug hints at the spring loaded collar. To release the tension on that spring tip at the upper left, pull the collar (using the ribs) down toward the P70/P75 planar while pulling the cable back from the planar.

External Storage Device Cable Pinout
   Do you want to get access to the DB37 header conductors? Unscrew the splined nut where the cable meets the DB37 shell. Pull the nut back along the cable, then simply pull the plastic shell after it. Now you can see that the metal hood inside is a two part deal. Note that there are two little latches on each side of the hood at the front, next to the hex posts. Use a fine bladed screwdriver or a knife to pop the hood open, one side at a time. 

Hosiden Connector to 37 pin Connector Pinout
   Now THIS is something I personally tested with the P70 to 4869 5.25 External Floppy. Note that this is the end of the 23F2716 cable!

Pin 10 on the DB37 connects to TWO pins
NC means there is no connection.
Shield to shield


External Storage Device Cable Source
  Search for 23F2716 or 23F3230 on the net.

P70 Trivia (applies to P75 some)
The system I am using sometimes refuses to start after power-on without any display or beep symptom. After several power on-off eventually it starts. Any idea???

From Peter:
   Looks like a slightly buggy power supply -or- systemboard. The PSU is a little sensible against aging and overheating after many years, because it is a known "dust collector". The PSU fan sits awkwardly mounted over the PSU and only blows half (or less) through the PSU, which causes dust particles to stay in the PSU. I would recommend that you remove it from the machine and give it a good blow with a compressor and air-nozzle to get out the dust bunnies from it. Make sure the fan is in working order. 

On various occasions I saw failing mainboards. They refused to come on - and I don't know why. They simply died from one day to the next and could only be replaced. Back in the "good old days" with full IBM support and machines under warranty that was no big problem. Today things look different. If cleaning the power supply doesn't help - and if removing all adapters and testing with the bare sysboard doesn't change the misbehavior: suspect the mainboard to be damaged and look for a replacement. 

Under rare circumstances a poorly seated card-riserboard caused trouble. Have the small two-slot riserboard removed, clean the contacts and spray a good amount of TV-tuner cleaner in the board connector. Let it dry for 10 minutes and use a piece of stiff card pushed into the connector with one edge and move it up / down the connector. Then spray the connector once again and let it dry for an hour. You might also repeat these steps on the two MCA slots on the riserboard. Especially on the slot pointing towards the top of the unit, which collects dust when left with no card installed for a longer time. 

Some older units also developed a sort of "creeping short" along the FDD cable where the FDD cable enters the "flip out" FDD bay. There was an ECA for the FDD cable from IBM for the earlier models, where FDD cables broke, shortened DC-voltages to GND or FDD signals and caused a dead systemboard. (Few only - IBM fixed it in an early stage). 

Another potential problem has been reported with the memory modules. The system takes only 1 and 2MB modules anyway, but certain 2MB modules installed on the sysboard connectors with double sided 10 chips layout (20 chips total - 2 x 8 data + 2 x 2 parity - 16 bigger and 4 smaller chips) caused overload of the memory driver lines. This usually resulted in either a 200-something memory error -or- a machine failing to come up at all. 

I take from the 16MB you'd mentioned that the upper slot contains a memory card (the lower is too short anyway). So you should probably check the card connector as described further above and check if you cannot use memory modules with lower amount of chips. Usually the memory installed on cards causes a 200-series memory error in first place before dumping the machine to total darkness - but you never know. 

I have several P70 in my collection and all of these run with double-sided 2 MB chips ... but I had a customer machine in '93 or so, which didn't. We used later 2 MB/70 ns 9-chip modules "hardware-recoded" (with soldering) to 2 MB / 85 ns and the system worked like a charm.

Remove Power-on Password (P75 shown, P70 similar)

To remove an active, unknown, power-on password do the following.
   1. Power-off the computer and unplug the power cord.
   2. Remove the system-unit cover.
   3. Short the two pins of JP1 together. 

With the pins shorted, power-on the computer. This erases the power-on password. Remove the short after POST is finished.

Content created and/or collected by:
Louis F. Ohland, Peter H. Wendt, David L. Beem, William R. Walsh, Tatsuo Sunagawa, Tomáš Slavotínek, Jim Shorney, Tim N. Clarke, Kevin Bowling, and many others.

Ardent Tool of Capitalism is maintained by Tomáš Slavotínek.
Last update: 08 May 2024 - Changelog | About | Legal & Contact