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Say we have a terminal (for example: VT100) that is connected to a computer (for example: PDP-11), and UNIX V7 is installed on the computer.

The terminal will have a serial port that is connected though a wire to a serial port on the computer, right?

Now could we change the baud rate for these two serial ports, or is the baud rate fixed?

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The short answer is yes. You can change the baud rate of a VT100 talking to a PDP-11.

On the VT100, this is simply done using the Set Up facility. This is entered using the Set Up key on the top left of the VT100 keyboard. This takes the VT100 offline and into a configuration mode. Then press 5 to take you to a screen where you can change the baud rate by moving the cursor to the appropriate value and using space to cycle through the available values.

This is described in more detail in the online VT100 Manuals in Chapter 5. Note that the input and output speeds can be set separately. This was useful in the days of split-rate modems like the CCITT V23 standard used for services like Prestel.

At the PDP-11 end, the baud rate was usually set with jumpers or DIP switches on the serial interface board. To describe how to do this would depend upon which serial card you have. Many later machines would auto-detect the baud rate from the attached terminal - just tap a key a few times and the PDP-11 would configure itself. However, this often didn't work with split baud rates as it is only looking at incoming data.

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    On the PDP-11 end, the console serial port was usually set with jumpers or a dip switch, and the old cards like the DL11 & DZ11 used jumpers. However, many of the later PDP-11 serial options had software programmable baud rates allowing someone running Unix to use ioctl(), 'getty' or 'stty' to set them. Examples of such cards would include the DZQ-11, DHV11 & DHU-11. I'm pretty sure the async SLUs (other than console) on the PDP-11/8x & 9x were software programmable as well. And there were many 3rd party async cards that did this as well. – KJ Seefried Oct 31 '17 at 22:21
  • The DZ11 had programmable line speeds. Likewise its predecessor, the DH11 mux. DL11 speeds were determined by crystal selection and (rotary) switch. – dave Sep 14 '18 at 2:34
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If the DTE and DCE are physically connected (i.e. a serial cable between them) and those endpoints support changing speeds then the speeds can definitely be changed mid-stream. The example would be two terminals on either side of a null modem. If the terminals both reconfigured from, say, 9600 baud to 19.2k the session would continue (..possibly with some transitory loss if the switchover wasn't perfectly simultaneous).

It gets more complicated if there are modems involved. With older, lower-speeds it was possible to change baud rates mid-session if the modems were hardware switchable. As an example, two terminals connected to 300/1200 (or 110/300) baud modems where rate was selected from the front panel could change speeds mid-session if the terminals were software switched or automatically detecting.

For higher speed links (9600 baud+) things got much more complicated, as the modems were constantly renegotiating based on line conditions - not to mention often switching rates depending on who was transmitting (many modems were 9600+ in one direction and 300-450 in the other). These modems tended to lock the speeds toward their DTE's, though - usually at a much higher speed than the modulating side could theoretically support. Changing terminal rates in that situation was a non-starter.

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