Telephone Switching > Electro-Mehanical Key Systems (1A1, 1A2 etc)

1A, 1A1 & 1A2 Key System Differences

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What is the difference between these types of PBX?
What is required to operate them?

I am in no way the expert here.  I have two 1A2's and none of the earlier systems.  Here is what I understand, and there are several others here that know a heck of a lot more about them than I do.  So if I am putting out anything wrong, I welcome any correction.

1A and 1A1 and 1A2's are all key systems, rather than PBX's  In fact, it is not uncommon for key systems to be running behind a PBX.

1A key systems are the very early version, and is all relay with no active electronic components, and has a different method of placing calls on hold and releasing the hold than the later versions.  A phone has to have a key mechanism that is just for 1A to operate, and I don't believe 1A key sets (phones) will work on 1A1 or 1A2.

1A1 and 1A2 have the newer "A" lead control for holding/releasing.  1A1, I believe is still all relay controlled with no active electronics.  1A1 and 1A2 keysets I believe are compatible with each other.  1A2 uses transistors and more sensitive components.

It's true that the early 44x and 46x phones work only on 1A. Later model 44xs and 46xs (such as 464GG) as well as 564s can be used with 1A, 1A1 or 1A2.

1A2 uses plug-in line cards, one for each line.

Here's an example of 1A Key showing two 1A Key Telephone Units=a hold relay and line relay for each line.

Here is one of the latest versions of 1A2: a Western Electric 620A panel for 8 lines. It currently  has 4 line cards.


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