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There are versions of the Atari 520ST, and perhaps the 1040ST, that differ in having many discrete inductors attached at the I/O port lines for serial, parallel and floppy ports.

The first image below shows a motherboard without inductors on J3 (floppy), J4 (serial), and J5 (parallel) ports. The second image is also a 520ST motherboard of a different revision that has the discrete inductors for J3, J4, and J5.

Are the inductors unnecessary, and were removed to save cost? Or was this a correction that added the inductors to solve some complicated line noise/standing wave or similar "complicated EE problem"?

520 ST motherboard without inductors

520 ST with inductors

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  • The dozens of orange things in the second photo close to the connectors? Are they not ceramic capacitors?
    – Michael
    Commented Aug 17, 2021 at 19:13
  • @Michael They are labelled with an "L" prefix on the Atari ST's PCB and schematic, which usually indicates an "inductor". And they are obviously not normal ceramic capacitors when viewed up close.
    – Brian H
    Commented Aug 18, 2021 at 1:39

2 Answers 2

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There are versions of the Atari 520ST, and perhaps the 1040ST, that differ in having many discrete inductors attached at the I/O port lines for serial, parallel and floppy ports.

Essentially all later ST have them. The first picture shows a C070115 Rev. 2 ST board wich is about the oldest in general availability. The second is a C070243 Rev. C which is the direct follow up (*1). This and all later 520 models had them added.

Are the inductors unnecessary, and were removed to save cost?

It's rather the other way around, as they were inserted later.

Are was this a correction that added the inductors to solve some complicated line noise/standing wave or similar "complicated EE problem"?

The chokes are to reduce EMC insertion as well as emission. Atari seemed to be very afraid about noise insertion at that time and it may be past of lesson learned from early units. Just open any Floppy and you'll note as well a way larger number of chokes than one would expect. Not to mention additional ferites.

A friend of mine jokingly called them Angst-Drosseln (*2) as he assumed them to be more meant to throttle the designers Angst about EMC than real EMC.

*1 - There were inbetween designs, but I'm not sure if any o them went into production

*2 - Drossel is the German EE term for choke/throttle but also meaning strangling (as in Erdrosseln). In a play with German Grammer the second meaning might be translated as "dampening the fear" :))

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  • The usual English/American term is "Noise emissions" not "insertion". It is probably just a native language difference but means the same.
    – Kevin White
    Commented Aug 16, 2021 at 18:19
  • @KevinWhite Erm, noise emission is about a device creating noise. Insertion is about a device being influenced by noise. Which was exactly the (an) issue with the ST disk drives - seeing magnetic flux where none was.
    – Raffzahn
    Commented Aug 16, 2021 at 18:23
  • Ok, I misunderstood. The usual English/US term for that is "susceptibility". It is very uncommon to have susceptibility issues and they are not usually tested for. More typically designs have problems with emissions when they are FCC tested and inductors, capacitors or extra shielding have to be added to meet the regulations.
    – Kevin White
    Commented Aug 16, 2021 at 23:04
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    Those issues are usually well tested, although more often than not by end users, like it with the early ST. These tests are usually acclaimed by shouting "That stupid floppy eating my disk again". FCC (if at all) only cares for emissions, users care way more for the other way around, isn't it?
    – Raffzahn
    Commented Aug 16, 2021 at 23:34
  • @KevinWhite EMC immunity (as it is called in the IEC standards I'm familiar with) is commonly tested in EU, and is quite important in order to tolerate emissions from e.g. mobile phones.
    – jpa
    Commented Aug 17, 2021 at 12:23
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They are not really inductors. They are EMI noise filters for suppressing electromagnetic interference that conduct out of the unit via the wires. These kind of EMI filters usually have two ferrite beads and a capacitor in a T configuration, to filter both incoming and outgoing interference.

The type of the filter is ZJS5101-02 according to ST manuals. Although it might be a typo, and the actual part might be TDK ZJS5101-102.

They would most likely not be there unless they are needed. They might be needed for example to pass official electromagnetic interference tests so that the product can be sold.

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