Hardware informations about the Floppy-Speeder Professional-DOS, Version 1
This section mainly describes the hardware of the
Floppy-Speeder Professional-DOS from Mikrotronic.
Since it didn't work, when it arrived, the whole hardware
was analyzed systematically.
In a first step the EPROM of the drive expansion PCB
(27128, 16KB) was read out and some interesting strings were
The drive identifies itself as:
0x05b7: SPECIAL DOS 1541
On another location of the additional Professional-DOS ROM
expansion area, this was found:
0x35d0: PROFES- SIONAL 1541 DOS
These contents lead to the asumption, that this piece of
hardware would probably be one of the very first revisions of
Professional-DOS or perhaps a prototype of it.
0x35f0: WAS MADEWITH THEGENEROUSHELP OF:
0x3610: UWE STAHL
0x3630: FRANK THOMAS MATTHIAS
0x3650: KEGEL- MANN AFTER AN
0x3670: IDEA OF KLAUS H. ROREGER
0x3690: IN 1985.........SEND US
0x36b0: YOUR IDEAS FOR IMPROVAL........
0x36d0: USE @D- TO MAKE USE OF RAM A000
0x36f0: TO BEFF........
⇒ Find more ROM informations at the
ROM and tools section of this
After retrieving the Professional-DOS hardware, the first
task was to documentate the hardware by some high quality
scan pictures. This helped reassembling the board, after all
work was done.
The following list describes the steps, that were done to
analyze and repair the board:
Now the drive board itself was working again. Professional-DOS
didn't reach the proposed speed yet, but that may be caused by
the fact, that the desired C64 kernal ROM replacement was
All socketed parts have been removed and the complete
board was measured out pin by pin (264 pins with a simple
multimeter). This way a complete crossconnection table
could be created that ended up in a schematic of the
hardware. It was a little bit difficult to decide, if
there's a connection between some of the pins, because
most of the TTL-IC's haven't soldered out yet. Sometimes a
resistance became nearly 0 Ω without a
direct connection between these two points. This way the
fault with the board could be found, because one of the
pins of the processor (address line A12) wasn't connected
to any other, but logically had to.
This procedure was a lot of work, nearly 14 long and
boring hours. Although the fault was found, the rest of
the planned procedure was done nevertheless.
All parts have been soldered out, so that some scans of
the empty PCB and the bottom side of the parts could be
made. As you may know, normally the labels of all the
TTL-ICs of Professional-DOS are removed to prevent users
from copying the hardware design (as happened with several
other speeder systems). Sometimes IC-producers print the
type number onto the bottom side of the chips and so it
was with one of the smaller ICs. This chip could be
identified as a 82S123, which is a 256 Bit PROM
(32x8 Bits) also known as 74S188. It seems, that it is
used as some sort of programmable logic (5 inputs, A0...A4
and 8 outputs, O1...O8), like a PAL or GAL.
All the circuit paths of the PCB have been measured out
again. This way the crossconnection table could be checked
again and some bugs were removed (wrong numbering because
of the many ill oriented parts). This check gave the
assurance, that there were not any more faults with the
board than the one found earlier.
The whole hardware has been reassembled then, but using
high precision sockets all the way. This way each part and
the whole board can be analyzed again without the need of
desoldering the board anymore.
⇒ View the documentation steps of disassembling the
hardware at the
picture gallery of this area.
Because another Professional-DOS user asked for help to repair
his hardware a deeper look into the hardware was needed.
»Someone needs to understand a thing, before he can
repair it«. First all of the unknown IC part numbers
needed to be unveiled.
After these steps some schematics were created, who brought a
higher level of understanding of the inner working of
Professional-DOS. There's not only the clock frequency
doubling system, but some circuit, that is able to
automatically divide GCR nibbles, buffer one of the nibbles
for one work cycle and decode them via the main ROM without
the need for extra 6502 code.
Two other TTL-ICs were identified, they are both the only
14-pin ICs on the board. The first was simple, because a
picture of Professional-DOS in the german C64 magazine
64'er showed a 74LS00. The second one was more
interesting. By simply spitting onto the top side of the
chip (iiieeeehhh :-) some rests of the labeling could
identify it as a 74LS74.
After playing around with a picture manipulation program
to prepare the scans for this web site another one of the
removed labels of one of the TTL-ICs could be identified.
In combination with the crossconnection table, it would be
safe to say that this chip is a 74LS173.
In the meantime all of the unlabelled ICs could be
identified as 82S123/74S188, 74LS123, 74LS00, 74LS157,
74LS367, 74LS74 and 74LS173. Each part was tested with
a prototyping socket board, some switches, bus drivers
(74LS245) and LEDs. An ASCII chart with all known TTL-ICs
was very helpful for finding the correct IC types.
Also the contents of the 82S123-PROM were read out
⇒ The results of these steps are collected on the
Electronics section of this
Later a friend found a little tool named
»PROFESIONAL 1541« in his C64 collection.
It creates Professional-DOS kernal ROM replacements out of
ROMs. After building such a C64 kernal and flashing it into a
new ROM, the Professional-DOS speeder system reached it's full
transfer speed. 202 Blocks could be loaded in less than 5
⇒ Get it from the
ROM and tools section.