Welcome to the webpage dedicated
to the H(igh) L(evel) S(ystem).(please wait a moment for the pictures to be loaded)
The page can roughly be divided in two sections. The first one is with the photo's of the ceremonial switchoff of the machine, the second is a general description of the hardware and the way it worked. Most pictures are quite small when you look at them but if you click on them you will get a larger version. If you want any of them in a higher resolution let me know, I have high resolution versions of them available.
For any questions or comments please do not hesitate to contact
The history of the machineFor many years the SITA network was based on a by now hugely outdated system called the HLS or to the people who worked with the machines as 'The Philips'. A complete name is a Philips DS714 or DS790 with added to that a Mark I, II, III , IV to indicate which specific version this one was.
Somewhere in the sixties it was decided that the switching of the traffic at that time (all telex traffic) had to be automated and eventually this resulted in the design and building of the Philips DS714 which was at that time definitely the most advanced system ever built. With a number of these machines the first real packet switched network was built by SITA called the HLN (High Level Network). There were typical speeds in this network of a whopping 2400 (!!!) bauds on international circuits.
Late 1980's this DS714 was decommissioned and a new version came in, the DS790. This was basically the same machine but built with more modern materials and technology. Yet, the original program of the DS714 could run on the DS790 with only minor modifications.
The DS790 has worked for over 10 years and one of these machine is currently still running in Singapore. In may 2000 it was decided to switch off the Philips in Amsterdam and we decided to put some commemorative sita on the web for it.
The machine is way to big to be kept as a souvenir or so but I am happy to say that large parts of it will go directly from the computer floor into a to be started museum so this magnificent piece of computer (and SITA!) history will not be lost completely.
The working of the systemThe whole idea of the system was that people can send (telex) messages around the world and be absolutely sure that these messages would be delivered. For this a secure 'store-and- forward' system was needed. To ensure that the HLS would store every single message several times. The first storage was on a tape unit and a drum as incoming message. Then it would be processed and after that stored again as outgoing message on tape. The tapes were kept for at least 7 days so everybody could get them repeated in case something went wrong with the (primitive) telex machines. The storage was only there to enable customers to ask for an investigation in case a message was not delivered, delayed or there was something strange going on like fraude with the addressing.
The machine was built up of a processor with a number of control and
expansion systems around it. For 'internal' storage there were two
separate systems, the 'drum' and seven tape drives. For external storage
there were a number of PC's (the retrievals) with SITA's own software
running on them.
The original DS714 did not have anything like a console or such. The
controls were two fold: One an old style Teletype and the other the front
panel. The teletype could be compared to a very old fashioned electronic
typewriter, the front panel looked like:
The 'new' DS790 was a revoluntionairy machine in that the teletype and
the frontpanel were no longer there, they had been replaced by a
Hewlett&Packard terminal! On the terminal you could see a
representation of the frontpanel and the keyboard acted like the old
teletype system. The screen looked like this:
The console was used for programming, starting and stopping the machine. For the day to day work the operators had a range of PC based control systems. The software for this was also known as the PP series. The reason for the abbreviation PP was never completely clear but it was rumoured this had to do with the fact that no budget was available for it so the software was written as a 'Pirate Project'.
The screen was divided into 9 windows:
Window 1 - Status of first system - Latest startup time, Software revision, type of startup
Window 2 - Status of second system - Latest startup time, Software revision, type of startup
Window 3 - Status of the subsystems like printers.
Window 4 - Status of the MIOM's and the MXT
Window 5 - Status of the 7 tape drives
Window 6 - The oldest two messages in the system. As far as I know it was only one single time that the system the status 'No messages in system'.
Window 7 - Status of the internal counters of the processor
Window 8 - Status of the retrieval machines
Window 9 - Status of the line monitors.
There were a number of other PC based system connected to the DS790. They were:
Besides these PC based control systems the HLS was the host system for
a new range of products called SITATEX.
If the MIOM went down there was a separate startup procedure which had to be followed. This was done with a special 'keyboard' with it's own paper tape reader:
The machine had two cables running to a special card which could be inserted into the MIOM. Then the MIOM could be activated and the program on the papertape could be loaded.
Looking at the processor cabinets you would not see very much:
However, inside these 4 cabinets a lot of things were going on.
Note the left picture, the small grey square in the bottom right is the
actual memory, the rest is just supporting electronics.
The system had 7 tape drives, two of these were in
constant use and needed to be able to start the system. These were the
statistics tape which was used to keep trackof which company sent how many
to messages to which destination. The other was the history tape needed to
be able to keep the 'store-and-forward' system running.
Specially the self loading tape drives had to know where the start of a
tape was. This was indicated with a BOT (begin of tape) which was a little
piece of reflecting material which was put a certain distance from the
physical end of the tape
On top of the tapedrives you could find a wooden block which black
labeling. The operator in the operations room could see the tape drives
and with the aid of these blocks he could in a glance see what each type
drive was doing:
To clean the drive's read and write heads, bottles of pure alcohol
could always be found in the area of the tape drives. This stuff was very
much not for drinking but it was used for all sorts of cleaning: