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Thread: WiFi Modem for VT-100/102?

  1. #1

    Default WiFi Modem for VT-100/102?

    I recently acquired a VT-102 in good working condition. I was thinking about connecting it to a Pi running a PDP emulator but I think some level shifting is required to get RS-232 working on the Pi.
    Then I wondered if there was a decent WiFi modem I could just plug into the VT-102 serial port and then I could just telnet to my Pi or maybe other systems out on the internet.
    A quick google search reveals a number of WiFi modem options but also leaves me with some questions.

    A VT102 manual I found says that the serial port is RS-232-C, is that so? or is it possible it's a variable that depends on the model or options?
    RS-232-C is 5V? So level shifting may not be necessary with a USB to serial converter on the Pi?

    Any recommendations for a specific WiFi (or Ethernet) serial to Telnet device? Are there any that can do SSH?

    Thanks
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  2. #2
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    Quote Originally Posted by Hutch View Post
    A VT102 manual I found says that the serial port is RS-232-C, is that so? or is it possible it's a variable that depends on the model or options?
    RS-232-C is 5V? So level shifting may not be necessary with a USB to serial converter on the Pi?
    RS-232 is bipolar, with a "1" being at least three volts positive (the specification says it should be 5-25v, 12v is commonly used for PC serial ports) and a zero specified as -3v (or more, again, -12v is common). A "proper" USB to RS-232 adapter (vs. a TTL serial adapter) typically uses something like the MAX232 chip that has internal charge pumps to create the correct signalling voltages off a 5v or 3.3v supply, so if you use one of those you should be fine driving the terminal with it. If you want to try to drive it directly off the serial pins on the Raspberry Pi's GPIO header then you *will* need to build your own level shifter using something like a MAX232.
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  3. #3

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    As far as the TV-102 goes, Isn't the RS232-C standard TTL? It's actually hard to find specifics on the different versions of RS232.
    I'm thinking if I get a TTL level RS232 wifi modem, it may just work with the VT-102 out of the box.

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    Quote Originally Posted by Hutch View Post
    As far as the TV-102 goes, Isn't the RS232-C standard TTL? It's actually hard to find specifics on the different versions of RS232.
    I'm thinking if I get a TTL level RS232 wifi modem, it may just work with the VT-102 out of the box.
    No, RS232-C (aka ANSI/EIA-232-E) is not TTL levels but rather +3V..+25V and -3V..-25V levels.

    Probably the simplest solution is to use a standard USB-serial converter cable with a DE9 to DB25 pin converter and plug it into your RPI, and the DB25 end into your VT102. This should produce a /dev/ttyUSB0 entry on your RPI. Then you need to enable a the RPI to spawn a login prompt over the serial connection.
    Ref: https://www.raspberrypi.org/forums/v...getty#p1129209

    So you should then be able to login via the VT102 to your RPI, and then SSH anywhere thru it.

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    Quote Originally Posted by Hutch View Post
    As far as the TV-102 goes, Isn't the RS232-C standard TTL? It's actually hard to find specifics on the different versions of RS232.
    I'm thinking if I get a TTL level RS232 wifi modem, it may just work with the VT-102 out of the box.
    "C" dates from 1969, and all subsequent revisions mostly just pick around the edges in terms of formalizing timing and making the standard D-shell connector an official recommendation instead of a serving suggestion.

    I don't know why you're so worried about TTL; if you get a wifi modem that has a DB-9 connector on it, of which there are many available, it's almost certainly going to have a MAX232 in front of the port. TTL serial devices generally come to pin headers and are *not* labeled RS-232. Or at least they shouldn't be.
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  6. #6

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    Quote Originally Posted by Eudimorphodon View Post
    ... I don't know why you're so worried about TTL; ...
    I'm mainly worried about not having enough signaling level to work with the VT102 if it's expecting 12v and only getting 5 and/or frying the TTL input with a 12v signal.

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    Quote Originally Posted by Hutch View Post
    I'm mainly worried about not having enough signaling level to work with the VT102 if it's expecting 12v and only getting 5 and/or frying the TTL input with a 12v signal.
    You CANNOT use TTL levels with the RS232 connector on the VT10x. It will be RS232 levels only (+/- 3V up to 25V). It is NOT TTL levels.
    You have to use a standard serial port like on a PC or on a USB/serial converter cable. The DB9 connector uses RS232 levels.
    Forget about TTL serial levels. They are the wrong polarity anyway (inverted H/L) from what RS232 levels are.
    RS232 inputs require a voltage that is a minimum of about +/- 3V and will accept an input up to +/- 25V per the spec.
    Typical RS232 output drivers today use either +/- 5V, +/- 12V, or +/- 15V on older equipment. Higher output voltage can drive a longer cable.

  8. #8

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    OK. So forget about TTL levels.
    I'm mainly a Commodore user and most of the devices I've seen are for the old 8bit Commodore machines which are all TTL level, so I just wanted to make sure I got a device that is compatible with the VT1xx.

    So how about it? I can't be the first one to have this idea. Is there a good RS232 WiFi device that others use? Something like a WiFi232, but that can do SSH?
    Or am I just better off using a Pi? I want to try out running the PiDP11 anyway.

    Thanks

  9. #9
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    Quote Originally Posted by Hutch View Post
    OK. So forget about TTL levels.
    I'm mainly a Commodore user and most of the devices I've seen are for the old 8bit Commodore machines which are all TTL level, so I just wanted to make sure I got a device that is compatible with the VT1xx.

    So how about it? I can't be the first one to have this idea. Is there a good RS232 WiFi device that others use? Something like a WiFi232, but that can do SSH?
    Or am I just better off using a Pi? I want to try out running the PiDP11 anyway.

    Thanks
    I would just start out using a standard USB/serial cable on your RPI and setting up the RPI to spawn a login shell on that line (see my post #4 above).
    You could run SIMH from the command line and have your VT102 'talk' to a PDP-11 (or PDP-8 or VAX). Or ssh to some other system.

    THEN when you get tired of that find an RS232/WIFI/SSH/TELNET adapter. You will likely end up paying 2X for that over what an RPI plus USB serial cable costs.

  10. #10

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    RS232 to TTL converters are quite cheap on Ebay.

    I think the most common chip is the MAX232 to convert RS232 to TTL levels.
    And remember, the voltages are inverted.
    So a TTL 1 is -3...-15V on the RS232 side. And a TTL 0 is +3...+15V on the RS232 side.
    The voltages between -3 and +3 are undefined.

    But for connecting to a Pi I would also suggest a USB to RS232 converter.

    Regards, Roland
    Last edited by Roland Huisman; November 15th, 2019 at 11:55 PM.
    WTB: Case for Altair 8800 ...... Rolands Github projects

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