Siemens PC-D

One of the first European IBM PC-compatible (at least in software) computers were produced by German Siemens company. First, a PC-X station has been made, with very high-end configuration. With 80186 processor running at 8MHz, there was 1MB of RAM and 10 or 20MB hard disk. Disk interface was SCSI, but high prices of these disks made Siemens use converters to connect MFM drives. A special Unix-like SINIX operating system was running on it. In 1986, two years after releasing of PC-X, Siemens started to sell PC-D, a stripped-down version od PC-X for home or office use. In the same time, high-end configurations for industrial control and multi-user systems have been sold in WS or WX line, they were PCs in casings styled like PC-MX, but even larger. In PC-D, mainboard has been re-designed to use MMU in flat package, there was no NVRAM and machines usually had 256 or 512kB of RAM. Most configurations had two 5.25" floppy disk drives and no expensive hard disk. A specialized graphics card with proprietary monitor could output even monochrome graphics to work with GEM environment in 640x350 resolution.
But in its hardware it was not compatible with PC at all. The system is oriented around VG96 bus, which is purely local invention incompatible with ISA or similarly looking British solutions (LSI). Picture is sent to display by 25kHz composite signal. Keyboard is supported by serial port while mouse is based on differential phase signals (my universal one should work).

Manufacturer Siemens

Origin Germany (West)
Year of unit 1986
Year of introduction 1986
Class XT
CPU Intel 80186
Speed 8MHz
RAM 256kB
ROM BIOS, monitor
Graphics Proprietary, 640x350 mono, 80x24 text
Sound PC Speaker
System expansion bus VG96 local bus
Floppy/removable media drives 2x 5.25" floppy disk drives.


Hard disk: Not present, possible upgrade with SCSI drive or converter card. By default NEC D5126 (20MB) or BASF 6188 (10MB).



Peripherals in collection:
 - Original keyboard
 - Original mouse

Other cards:


None, 2 additional slots present.
Non-standard expansions: Possible to add memory up to 1MB, SCSI hard disk?
Operating system(s): MS-DOS 2.1

The history of my unit is not known. It was sold to me as "floppy disk drive for Siemens WX-10 or WS-10", so probably it was used as terminal. Keyboard and incomplete mouse are from different sets. As it came, the machine had 2 RAM chips defective. It was repaired and awaits a working monitor.
I got also a monitor but for Siemens WX/WS line, so it's colour one and it's in quite miserable condition - converter doesn't start and deflection are bad. Here is the photo of it, if you want.

Contents: Starting, diagnostics DIP Switches Pinouts Links


After successful POST it shows:

* * * TEST END * * *

And starts to look for boot disk in drive 1 (usually one in the right), then Drive 2, if there is a hard disk then it tries to boot from it. If there is no bootable disk in drive and no hard disk to boot from, then it waits about 5 seconds and retries procedure.

The machine starts as most PCs, preforming power-on self test called PUP (Power-Up Prufung?). This test is performed before display is initialized. After the most important tests, red LED in front panel is turned off. In front panel, there is a row of 8 diagnostic LEDs on the right of Power/Disk LEDs, below Reset/Debug buttons. You can see it in the photo as a long slot under "R D" markings. These LEDs light up in combinations when particular test ends. Service manual gives the following errors:

* - LED ON, - - LED OFF, (*) - Orientation LED always ON.

Code LED state Description
- (*)******- Microprocessor did not even performed Reset. Does it work at all? Applies also if all LEDs are lit up.
0 (*)------- EPROM checksum error
1 (*)*------ Error in first 64kB of memory while testing data lines
2 (*)-*----- Error in first 64kB of memory while testing Address and bank switching.
3 (*)**----- Local bus or both interrupt controllers bad (test during Read/Write IMR)
4 (*)--*---- One or more Interrupt controllers bad
5 (*)*-*---- Programmable timer 0 bad
6 (*)-**---- Programmable timer 1 or 2 bad
7 (*)***---- NMI because of access error (read of not existing I/O address)
8 (*)---*--- Status register contents bad, NMI not ran properly?
9 (*)*--*--- Memory test fail, preformed from 64k to the end or except last 64kB if 1MB is installed.
A (*)-*-*--- MMU Error
B (*)**-*--- DMA controller error
C (*)--**--- Floppy disk controller error
- (*)*-**--- Error detecting USARTs
- (*)-***--- Error detecting video board

Now, 3 USARTs and video controller are tested. If there is an error in these tests, it is indicated by BLINKING LEDs, and there may be more than 1 error indicated:

LED State Description
(*)*------ USART1 (Printer port) error
(*)-*----- USART2 (Keyboard port) error
(*)--*---- USART3 (Serial port) error
(*)---*--- Video controller error (expansion board)
(*)----*-- NVRAM or SRAM error
(*)-----*- RTC error

If it passes to this level, display board can be initialized and error may be just printed on screen. If everything is OK, it should display "*** TEST END ***" message.

During tests, when unexpected interrupt occurs, test is halted and interrupt is shown in LEDs:

LED State Description
(*)--*-*- Overflow interrupt
(*)*-*-*- Division by zero, Single-stepping or INT-error
(*)-**-*- Unexpected interrupt from power monitoring circuit
(*)***-*- Masked interrupt occurred
(*)---**- NMI because of parity error. Usually one or more RAM chips are bad.
(*)*--**- NMI because of bus timeout.

Most memory problems will be indicated by the "NMI because of parity error" light, because if one bit in chip is bad, address/data tests will be halted by this interrupt.

Other errors may be indicated by steady lights, even during normal machine operation (e.g. when it hangs):

LED State Description
(*)--*-*-- Overflow interrupt happened
(*)*-*-*-- Interrupt caused by division error, single-stepping program or INT command
(*)-**-*-- Unexpected NMI or error caused by power problems (like Brown-out reset).
(*)***-*-- Masked interrupt happened
(*)*--**-- NMI from parity error or bus timeout

Boot errors - machine fails to boot and displays the reason:

LED State Description
(*)-****-- No bootable media found
(*)*****-- Error in loader format
(*)dd--*-- Error in Restore (?)
(*)dd---*- Drive Status error
(*)dd-*-*- Seek error. Most common cause is by bad drive (head alignment)
(*)dd--**- Error when reading filesystem
(*)dd*-**- Error when reading bootloader
(*)dd****- Unexpected interrupt

WHERE dd is drive number:
-- - floppy disk 0
*- - floppy disk 1
-* - n.a.
** - Hard disk

Machine displays this "*** TEST END ***" and then proceeds to boot from drive 0 (by default, on the right). It requires disk with "SIEMENS" string in boot sector (see service manual). If drive 0 is not ready, tries drive 1. If this is not ready too, it tries to boot from the hard disk. If it can't boot even from it, it waits few seconds and retries booting starting from first floppy drive.

Hard disk controller errors - applicable only to machines with hard disk:
- The HDD LED lights green when HDD is selected
- The HDD LED lights red, when HDD controller encountered an error. The LED blinks in 0.5s periods. Then there is 2s pause and the code (in number of blinks) is replayed:
1 - Power failure (no power to disk? Have you checked 5V too?)
2 - Motor speed >10% over tolerance
3 - Positioning (seeking) error
4 - Motor speed >1% over tolerance. Drive needs calibrating (Basf drive only).
5 - Power-on error (??)
6 - Motor not running while signal "MOTOR ON" announced.
7 - Write logic error

Hard disk was installed using special controller board installed on power supply unit. It was connected to mainboard with 50-pin ribbon cable, and offered capability to connect MFM drive. This is a SCSI interface with SCSI-to-MFM converter board. It is mostly pin-compatible with SCSI (only no termination power and /ATN signals), but software implementation is a bit different. Generally forget about running gigabyte-size drives there. It looks like they designed it for SCSI, but drives were too expensive and used MFM drives.

Ah, I almost forgot: Battery is a Lithium one, 3.5V, non rechargeable. It powers RTC, as NVRAM is in most cases not present. Socket for NVRAM is present near USARTs and PICs (Interrupt controllers).

The Power Supply Unit is a complex switching one with digital on'off and conditions feedback. The connector pinout is following:

| View from top, here is rear of M/B.
| 1  2  3
| 4  5  6
| 7  8  9
| 10 11 12
| <-Mainboard edge

1,2,3,5 - GND
6 - +12V
7,8,9 - +5V
4 - -12V
10 - Digital output, constant 0 if mains is not stable in frequency (?)
11 - Digital output, ?
12 - Input, short to GND to turn on.




DIP Switches

Memory switches
Near the memory chips towards rear of the computer, there are 3 memory switches S2 S3 S4. They look like small levers, not DIP switches. To close the switch, push the lever to make it latch on hook. 1 means switch is closed.

Capacity S2 S3 S4 Chips


1 0 0 36 x 64kBit (4164)
512kB 1 1 0 18 x 256kBit (41256)
1MB 1 1 1 36 x 256kBit (41256)

Near the corner on the rear, a block of configuration switches S5 is present.
S5.2 -> Open - SRAM/NVR is not tested. Closed - SRAM/NVR is tested. Normally open as there is no NVR.
S5.4 -> Not used
0111 - After self-test launch Monitor
1011 - Try to got to Monitor without self-test
1101 - Skip self-test, Load and start OS
1111 - Perform self-test, load and run OS. Normal operation.

FDC test switch S6 is located near the edge in the front of mainboard, opposite to memory, near the Floppy Disk Controller chip (WDC chip) and its circuitry. Normally opened. Do not close and put valuable disks in drive.




Important Pinouts:

Connectors location:

Video out (Monitor):
1 - N.C.
2 - +12V
3 - N.C.
4 - GND
5 - GND
6 - N.C.
7 - N.C.
8 - N.C.
9 - Composite

WARNING: Display is at 25kHz Horizontal and 66Hz Vertical. Typical TV is 15.7kHz Horizontal and 50Hz Vertical. This usually will not go.

Mouse pinout (my universal mouse should fit):
1 - Left button
2 - N.C.
3 - Xa
4 - Xb
5 - GND
6 - Right button
7 - +5V
8 - Ya
9 - Yb


Keyboard port pinout (out means Computer->Keyboard):
1 - Data IN, Positive
2 - N.C.
3 - Data OUT Positive
4 - +5V
5 - GND
6 - Data IN Negative
7 - GND
8 - Data OUT Negative
9 - +5V


V.11 ports pinout:
1 - Data IN Positive
2 - +12V
3 - Data OUT Positive
4 - CRS Positive
5 - GND
6 - Data IN Negative
7 - FEIN Negative
8 - Data Out Negative
9 - CRS Negative


RS232 ports pinout:
1 - GND
2 - TxD
3 - RxD
4 - RtS
5 - CtS
6 - DSR
7 - GND
15 - SCTDCE (NOT IN PRINTER PORT) - Send clock?
17 - SCRDCE (NOT IN PRINTER PORT) - Receive clock?

 20 - DTR
22 - RI (NOT IN PRINTER PORT) - Ring Indicator for modem
23 - DRS (NOT IN PRINTER PORT) - Data Rate Selector
24 - TC (NOT IN PRINTER PORT) - Transmitter clock





Links:  - Hard disk version in blog post.  - Look for translated parts of Service Manual. - In some other collection - Service Manual. Part II is mostly about diagnostics software.