The Zilog SCC in the early Macs could operate in either 232 or 422 mode. AppleTalk used the later.

422 and 423 differed primarily in that the former used separate return lines for each data line, while the later shared lines.

AppleTalk used three wires. Even PhoneNet only used three of the four available. This would tend to imply that it was "really" 423.

However, I am not clear on whether there were other differences between the two standards? Voltages perhaps?

Does anyone know for sure whether AppleTalk used one standard or the other?

  • 2
    As I understand it, AppleTalk was the full networking stack and could run over multiple physical layers, including Ethernet. I think you're talking about the LocalTalk physical layer here? Or are you talking about the physical protocol between the port on the computer and the interface box?
    – cjs
    Sep 20 '19 at 16:56
  • 1
    Why not "really" RS-485? 485 is nearly the same thing as 422, with the main difference being that 485 defines a multi-drop connection, whereas 422 defines a point-to-point connection. RS-485 specifies a differential pair plus a common signal ground. The signal ground isn't critical, and it can be left out in many short-distance applications, but it usually is present as a third conductor in longer connections. Sep 20 '19 at 17:05
  • @SolomonSlow You're right. It can be described as a 485. Then again, 485 covers are a wide range of incompatible implementations, so saying it's a 485 wouldn't be of much help either.
    – Raffzahn
    Sep 20 '19 at 18:33
  • I'm pretty sure PhoneNet only uses 2 wires. I used to run normal voice over the red and green, and network over the yellow and black in a standard 4 wire phone line.
    – Glen Yates
    Sep 20 '19 at 20:23
  • @GlenYates: Correct, PhoneNet really uses only two wires.
    – PoC
    Sep 21 '19 at 22:40

(For this Answer I assume your question is about LocalTalk interfaces, not AppleTalk as system)

TL;DR: If at all, the network side could be described as EIA-485.

I guess a basic problem is that not only all of these do have similarities, but LocalTalk in fact uses two different interfaces. One between computer and the little box, and the other between these boxes. Equally important, both of them are used in a proprietary way. After all, it's LocalTalk, not either of these standard interfaces.

The Principal Setup is formed by a serial interface from the computer connected to a little box, called 'Connector Module', which in turn on the network side has two sockets for cables to be connected to other connector modules.

On the Computer Side an EIA-422 (V.11) interface is used. But the lines aren't fed into a (digital) comperator within the box, but are simply joined via resistors in the RXD lines and feed to one side of a transformer. So strictly this is no longer an EIA-422 implementation, but something different, vaguely compatible - and only guaranteed to work with an SCC and as implemented by Apple.

On the Network Side signal lines of both connectors (Pin 1/2) were connected to each other, making them two uninterrupted lines, connecting all stations. For signalling the other side of the transformer was put between the lines. This is strictly not daisy-chaining like often stated, but parallel taping - much like on old fashioned yellow cable. When no cable was plugged on either side a 100 Ohm resistor was switched in as termination. All together forming a two wire bus being neither of the named standards.

So far the network side looks similar to EIA-485. Except the signaling levels as well as termination are different. Then again, most EIA-485 implementations differ not only by the protocol, but in implementation as well. It's more of a family of relatives than a plug and play product. Everyone who tries to connect an IBM POS printer to a Profibus controller will know :)


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