RS-232 Echo Local vs. Remote

Question

Is there a term used for whether a serial port echos characters received remotely versus having the local machine echo characters sent locally? I am looking to establish a SCPI command for turning on/off this remote echo protocol. Do most serial interface systems rely solely on echo characters locally when desired?

Answer 1

There are two different terminologies in play, that I'm aware of...depending on whether you're talking about a terminal device, or a host computer.

Back in the day....using a computer usually meant a VT-100 terminal or similar RS-232 device, connected via modem or hardline to the host. Such terminals would often have a DIP switch selecting between "half duplex" mode (which echoed characters locally, for devices that didn't support simultaneous incoming and outgoing I/O), and "full duplex" mode, where the remote device was responsible for echoing any input it received.

The Unix "stty" command can enable or disable local echoing from the host side of the connection. This was probably necessary to interoperate with legacy devices that were hardwired for half-duplex or full-duplex only.

You are somewhat likely to encounter either terminology (half/full duplex, host echo on/off) to describe this sort of configuration.

In the context of modern human interface devices, full-duplex/"host echo enabled" is by far the most common configuration.

Embedded devices, on the other hand, are quite likely to employ custom protocols that don't involve one side echoing its input back across the connection.

Answer 2

No

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When interactive systems with relatively or even absolutely dumb (what we now call) clients were common almost all of them used a round-trip to what is now called the server for character echo. It simply wasn't possible to have any control over the user experience without it. Even the purely electromechanical teletypes used full duplex on Unix systems.

The few systems with local echo were more-or-less emulating punch card input and generally were used to run a single large program that cooperated with the terminal hardware to run customer information systems or other online applications, but not true interactive computer access. Some of these systems were expensive and sophisticated, and they ran a lot of bank terminals, but they were almost universally disrespected for their interactive design.

As a concrete example, it is unlikely that even 0.1% of all the interactive Unix remote clients ever used local echo. (I'm sure it happened a few times on some pathetic and now forgotten mainframe.)

Having said all that, I'm not familiar with your application and perhaps it is simple enough that local echo makes sense. Since today's clients are far more powerful than yesterday's "servers", we pretty much only see the old paradigm when we ssh into a CLI to do some local system admin.

But even in that case in today's world we always have a "full duplex" (remote echo) system.

Answer 3

As far as I recall, the concept is called "local echo", and it it's never used anymore.

Answer 4

I've seen this SCPI command:

SYSTem 
   :COMMunicate
    :SERial
       [:RECeive]
          :ECHO ON | [OFF]