7

This is how I think terminals were connected to a PDP-11 computer:

A PDP-11 computer had only one port that you can connect the console terminal (/dev/console) to, and if you want to connect more terminals, you have to connect an add-on (that have terminal ports) to the PDP-11 computer, and then connect the terminals to the terminal ports on this add-on.

Am I correct? and can you post pictures of what a terminal port looks like, and what a terminal cable that connects to that port looks like.

8

There are two general ways to get terminals connected to a PDP-11:

  1. via Line Adapters or Multiplexors that provided direct terminal connections
  2. via a network link (using network protocols like Telnet, LAT, etc.)

Terminal servers like the DECservers (but others made terminal servers as well, e.g. Xyplex) use method #2 to allow locally-attached terminals to connect to network resources (potentially very far away, e.g. in another state or another country).

For direct connection, you would plug in a Line Adapter card (the exact type of card depended on which PDP-11 you had), and the Line Adapter card would then be connected either directly to the device, or via a port module that attached to the computer's back panel. The cards, modules, and connectors varied widely depending on which model PDP-11 you had, the kinds of devices you wanted to connect to, and the kinds of connectors you wanted. RS-232 connectors were very common but they weren't the only kind.

To get an idea of the options that were available from DEC, have a look at a PDP-11 Peripherals Handbook. There were also third-party options available, generally these were compatible with one of DEC's options.

For example, the DJ11 adapter lets you connect up to 16 terminals to a panel which is then connected to the adapter via a single ribbon cable. The 1976 Peripherals Handbook (linked above) has a picture of it on page 4-112.

3

The typical interface would have been a DZ-11 which would be an 8-line asynchronous multiplexer. This was a Unibus card that was connected via a ribbon cable to a backpanel. This had the familiar V.24 25 pin connectors on the EIA/RS-232 version. The panel also was an anti-static filter. This may be the second component you were thinking of providing the connectivity.

Local Area Terminal severs or LAT like the terminal server did exist but were not so widely used on PDP-11s (used more on VAXes).

2

Not quite correct.

The PDP-11, like all DEC machines, had one port which was dedicated to the console device. The dedicated console port had elevated privileges and was used by the system for event logging. This did not mean that it didn't have other serial ports into which you plugged the terminals.

You may be thinking of a DecServer. If your DEC machine had a LAN, then you could connect DecServers to it. These were terminal servers that connected to the LAN and provided a number of serial ports for the terminals.

DecServer front

The server was configured on the DecNet LAN, appearing as /dev/dsvr1, or similar. It could then be addressed and the terminals configured to appear as, for example, /dev/tty1 upwards. Peripherals could also be assigned, e.g. /dev/lpt1. The ports could be individually configured for baud rate, parity bits, etc. There were also the usual network tweaking type settings available; packet size etc.

It was quite normal for a DEC installation, be it PDP or VAX, to have a number of DecServers supporting a large number of terminals and peripherals.

The cables for the terminals were simply DB25 serial cables as the ports were usually RS232. 100mA current-loop DecServers were available.

  • Wyse was pretty big into terminal servers as well, before "terminal server" actually meant a PC running some software that made it look like another PC somewhere else ;) – tofro May 26 '17 at 11:41
  • 1
    @tofro Wyse was a latecomer to terminals, compared to Dec. My first video terminal (~ 1983) was a Wyse 100. Beautiful machine - solid metal case including the detached keyboard, WyseWord optional ROM, very ergonomic for its time. They didn't get into terminal servers until many years later (after making a LOT of Wyse 50, 60, etc. (the 100 was first but most of the others had much lower numbers)). They progressed to the general PC business and then into terminal servers, thin clients, etc. While there was some overlap, basically the PDP-11 era was winding down by the time Wyse got big. – manassehkatz May 26 '17 at 15:22
  • How many terminal ports does the PDP-11 have? – user7681202 May 26 '17 at 17:29
  • @user7681202 - "the PDP-11" is a family of modular computers, not a single model. It has as many terminal ports as you wish to install... – Whit3rd Jun 2 '17 at 7:25
  • I remember when our University computing facility installed the first LAT servers (these might have been PDP-11 machines, before the DECserver product) connected to our VAX. I kept feeling an ever-so-slightly increased delay in echo responses to typed characters, compared to direct serial lines. Of course, once our VT-100 and VT-220 terminals went away and were replaced with PC-based terminal emulators, their own delays made the difference unnoticeable. – Lawrence D'Oliveiro Oct 20 '17 at 2:56

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