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I have an Esprit 250c Modular Color RS/232 Terminal "Intelligence Unit"
( overview here, and pdf of features here )

esprit 250c

I would like to make it work as a text-based tty terminal, maybe VT100 as it seems to be one of the most common standards.

Here are the connections this device has:

device connections

I also have a PC with a 9-pin serial port that I have running Ubuntu 14.04 and access to PCs and Macs.

I have been looking into the utilities like putty, screen, minicom, kermit and so on. I also read up on the LTSP project and watched a few videos on how to create Ubuntu thin clients on YouTube.

I have been reading up on things like the Text Terminal How To which seems to be one of the most complete resources but I still have a few questions.

I'm very comfortable with Linux and understand the basics of how things like named pipes work and, and the history behind why terminals sessions are referenced as /dev/tty1 etc.

I get the impression that I'm doing the opposite of what most people are doing - namely using a terminal emulator program to connect to a remote "server" rather than trying to figure out how to create a "terminal text server" to power an antiquated device.

Nevertheless, I have several questions regarding the "next steps" of my project:

  1. I'm assuming I can run something like minicom on my linux box and connect the Esprit directly to that using the serial port. But...
  2. How does the "handshake" work? Do I need to configure specific settings on both devices to allow them to talk? ( baud rate, dialing string, etc. ? )
  3. Or once you plug in the serial cable is it really just as simple as if you pipe text to /dev/ttyS0 have have it show up on the monitor of the Esprit?
  4. If configuration does need to take place, do I need a special keyboard to access this? According to this chapter it seems like there used to be a few extra keys on most terminal keyboards, like the "setup" key, which are apparently no longer included in a standard 101-key keyboard)? If not, how do I press "setup" on a standard keyboard? ( I could not find any manuals with instructions for setup for the Esprit 250C )
  5. Does having a serial cable with more pins work differently from the smaller serial cable that has only 9 pins? Do serial ports with more pins work at a higher speed?
  6. This device came with a plastic adapter for the bigger serial port ( Serial 1 ) and on the back side is an RJ45 plug. It is labeled MODEM CAB-5MODCM. What is this for? It looks like it would allow you to do a serial connection via an RJ45 plug or RJ11 plug which I believe have 8 and 4 wires respectively. How many wires are used in the minimum RS232 communication? 6? 4? 2?
  7. Any suggestions on specific next steps or common pitfalls to avoid?
  8. If using something like an serial to RJ11 or RJ45 adapter, how do I know what pins to use? I've seen several of these with 'configurable pins':
    enter image description here
  • It is not serial to RJ.. both are serial. it is 9pin D sub-miniature to RJ.. – ctrl-alt-delor Jul 9 '14 at 15:18
  • I don't know why you would use RJ.. Your PC and the terminal device both use 9pin sub miniture D. You need a fully connected null modem. gnd—gnd, rx—tx, tx—rx, rts—cts, cts—rts, dtr—dsr/dcd, dsr/dcd—dtr, have a look here en.wikipedia.org/wiki/Null_modem#Wiring_diagrams – ctrl-alt-delor Jul 9 '14 at 15:29
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5,6) 9pins is usually enough, the extra pins are secondary data or extra hand shake. The minimum number of pins/wires is 3: ground (gnd), transmit(tx) and receive(rx). It is better to have more handshaking: clear to send and request to send, then a bit better is to add data set ready, data terminal ready. Note the biggest misunderstanding in rs232 is the terms data terminal and data set: a data-terminal is any terminating equipment (computer, terminal, printer, …), a data set is a modem or cross-over cable (null-modem), and nothing else.

2) Hand shaking is in 3 levels:

  • soft: special characters are used, this is ok for text, as they are not used in text. this only needs gnd, tx, rx.

  • hardware handshaking: clear-to-send request-to-send.

  • data-terminal-ready, data-set-ready: these allow the to devices to detect when they are plugged together.

1) I think (and I am less sure on this bit), that there is a command line option to the Linux kernel to tell it to use a tty as a login tty (same as the ttys on ctrl-alt-f1 etc). You need to add this option to the config of the boot loader (probably grub). I don't think you need to use minicom etc. see section 15.1 how to set up getty.

  • This is looking very promising! Say I am using a serial to RJ45 or RJ11 adapter - a lot of them have 'configurable pins' ( updated question with an image of these ) - how do I know which pins I use for gnd, tx, rx, etc ? – cwd Jul 9 '14 at 14:31

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