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inmbolmie / 5250_usb_converter

Converter to plug an IBM 5251 terminal to a Linux PC via USB emulating a VT52 terminal
GNU General Public License v3.0
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5250_usb_converter

Converter to plug an IBM 5251 terminal or in general a 5250 compatible terminal to a Linux PC via USB emulating a VT52 terminal

converter PCB

Changes (2023/03/18): Improved Teensy firmware and Python script. Binary firmware update file. Now compatible with IBM 5291 terminals. New daemon and login modes available. General stability improvements.

For more information refer to this thread in Deskthority.net that contains an in-depth description of the converter and protocols involved.

This converter only works with IBM 5250 compatible (twinax) terminals. Particularly, this is not IBM 3270 (coax) compatible, as that is a totally different product line of terminals from the same era but oriented for the mainframe market. Fortunately if you are interested in the 3270 equivalent of this project please refer to this other project

For any question or if you tried and worked/didn't work get in touch at this email address: inmbolmie [AT] gmail [DOT] com

The converter functionality is divided between two components:

The converter works in Linux systems and Windows 10 systems installing the optional Linux Subsystem component (WSL)

Compatible terminals

These are the terminals reported so far to be working with the adapter:

Included files

Board instructions

converter PCB

Fabrication

For making your PCB you have several options:

Programming the Teensy

You only need to upload the .ino file as is to a Teensy 4 using the Arduino IDE with the teensyduino addon. More information here.

Be sure to generate the firmware with the following settings:

It has been reported that newer versions of the Arduino/Teensyduino environment (Arduino 2.X and Teensyduino 1.57, and maybe others) generate slower code that does not work properly with the adapter. The reference versions I'm using are quite old, Arduino 1.8.13 and Teensyduino 1.53. In case of doubt generate the firmware using those versions.

I will include on the Arduino source directory a binary distribution (HEX file) generated with those versions that can be directly loaded with the Teensy utility. For that you have two options:

On-board terminators

The on-board terminators are implemented using a couple of 100Ω variable resistors. You have to adjust the resistance between the two connected terminals (left and top-right terminals as seen from the top) to 54,9Ω. Optionally you can substitute the variable resistor for an equivalent fixed-value resistor.

The on-board terminators are enabled installing the two JP1 jumpers (1 & 2). You have to disable them removing the jumpers only if you are using autoterminated T adapters (most DB25 to Twinax adapters are), of if your device is not at the end of the twinax cable chain. Both ends of the Twinax chain must be properly terminated for the converter to operate correctly.

Arduino pin assignments

Python script installation

No special installation should be required. You need to have the Python3 interpreter. Currently Linux is the preferred OS, but the script has been tested to work in WSL Ubuntu under Windows 10.

The only Python module you should have to install is ebcdic, if you don’t already have it you need to execute in your system the command:

$ pip3 install ebcdic

Running the script

Just run the script with no arguments if you only have a 5250 terminal at address 0 and you want to use the default keyboard mappings.

$ python3 5250_terminal.py

You can change the default terminal address editing the value DEFAULT_STATION_ADDRESS at the beginning of the script

Under WSL in Windows you obviously need first to install the WSL support and a Linux distribution. For that refer to http://docs.microsoft.com/windows/wsl/install-win10

To run the program under WSL you need at least to specify the right COM port to use with the -t parameter. Check in the Windows device manager the COM port assigned to the converter and then run...

$ python3 5250_terminal.py -t /dev/ttySX

...where X is the assigned COM port, for example for a converter connected to COM3 run:

$ python3 5250_terminal.py -t /dev/ttyS3

Specify other connected terminals

If you want to specify a different terminal address (say your terminal is listening at address 5) run instead:

$ python3 5250_terminal.py 5

That will look for a 5250 terminal at address 5 with the default keyboard mappings.

You can specify more than one terminal as parameters to the command, up to the 7 supported terminals

$ python3 5250_terminal.py 1 2 3

...will look while running for terminals at the addresses 1, 2 and 3

If you want to specify a different keyboard mapping of those available under the scancodeDictionaries section (more information about that later) you can specify it next to the terminal name separated by a colon :

$ python3 5250_terminal.py 0 1:5250_ES

...this will look for terminal 0 using the default keyboard mapping and will look for terminal 1 using the 5250_ES mapping defined in the scancodeDictionaries section

Some emulated 5250 terminals are very slow and need a longer poll interval to make them run faster. If you have such a terminal you can specify a “slow poll” mode separating it with another colon:

$ python3 5250_terminal.py 0:5250_ES:1

......this will look for terminal 0 using the 5250_ES keyboard mapping and with slow polling active (mode 1)

There is also a “very slow poll” mode that is activated specifying “2” as a mode value, this should only be used for debugging purposes as the terminal will be unusable in this mode.

$ python3 5250_terminal.py 0:5250_ES:2

An arbitrary minimum interval between polls may be specified by providing a value in microseconds with a us suffix, e.g.:

$ python3 5250_terminal.py 0:5250_ES:10000us

Note that the actual interval between polls may be greater than the requested value due to CPU load, scheduling latency, how quickly other terminals respond to commands, etc.

Finally you can specify for ASCII to EBCDIC translation a different codepage, cp037 is used by default but for example:

$ python3 5250_terminal.py 0:5250_ES:0:cp500

...this will look for a terminal at adress 0 using the 5250_ES keyboard mapping, with slow polling disabled and using EBCDIC codepage cp500 for character translation. You can only specify here codepages supported by the ebcdic Python module.

Remote access to command line

The program can be launched to accept TCP connections on port TCP/5251 for remote access to CMD interface. Use for that the -p parameter:

$ python3 5250_terminal.py -p

Then you can connect remotely to the CMD using:

$ telnet <host> 5251

Multiple simultaneous connections are allowed.

You can also expose the CMD using a Unix Domain Socket with the -u parameter:

$ python3 5250_terminal.py -u

Then you can connect to the CMD using:

$ socat stdio UNIX:/tmp/5250_cmd_sock

Debugging

There are also some additional parameters to control debugging. When you run the program it always generates a “debug.log” file in the current directory where the errors eventually generated are dumped.

$ python3 5250_terminal.py -c

The -c parameter will generate additional debugging in debug.log for every command sent and received. This option will generate a lot of log lines, so it is not recommended at all unless strictly necessary

$ python3 5250_terminal.py -k

The -k parameter will dump to debug.log one line for each key pressed in a terminal, like this:

RECEIVED SCANCODE: 0x2d FROM TERMINAL: 0

This mode can be useful to discover the scancodes generated by your terminal’s keyboard, to be able to configure a custom scancode mapping for your terminal.

$ python3 5250_terminal.py -i

The -i parameter will generate in the current directory a couple of files write.log and read.log where all input and output to the terminal shell will be dumped for debugging purposes.

Disable keyboard clicker

You can optionally silence the terminal keyboard clicker, having three ways to do it:

Specify different USB device

$ python3 5250_terminal.py -t /dev/ttyACM0

The -t DEVICE parameter allows to specify a different serial USB device for connection to the Teensy, in case you have more than one or it is for any reason in a device different from the default /dev/ttyACM0

Daemon mode and/or the login or shell executable

You have three command line parameters relevant for these modes:

The -d parameter allows to start the script in daemon mode. In this mode the script will run in the background. The log file is now written to /tmp/debug.log

This way you will be able to run the script on system startup with a command like this:

daemon --name="5250" -- /usr/bin/python3  ~/github/5250_usb_converter/5250_terminal.py -p -d

The -p parameter makes the script listen on TCP port 5251 for telnet control connections.

The -l or --login parameter specifies the executable to be run for every incoming terminal session. By default, etc/twinax_login_default is run, which starts the current user's login shell, passing it the --norc option if it's bash.

See the executable etc/twinax_login_* files for examples of scripts that may be passed for this option, including etc/twinax_login_custom_terminfo which uses a custom terminal definition to support use of the F1 through F12 keys if they're supported by the keyboard mapping.

Passing --login /usr/bin/login causes login prompts to appear for every incoming terminal session as seen for example on a Linux console. You may have to configure the shell yourself to set the environment variable TERM=vt52 and maybe configure a correct environment for bash as those cannot be managed through the "login" command as login simply runs whatever it is on /etc/passwd, although it should preserve the value of TERM set by 5250_terminal.py. Note that this mode is insecure as requires to run the script with root privileges, for example:

sudo daemon --name="5250" -- /usr/bin/python3  ~/github/5250_usb_converter/5250_terminal.py -p -d -l /usr/bin/login

Keyboard scancode mappings configuration

The keyboard of a 5250 terminal doesn’t directly generate characters, instead a “scancode” is sent back to the host for every key pressed. Those scancodes need to be converted to characters for the tty shell. Unfortunately there are a wide variety of possible combinations across the 5250 terminal range (5251, 5291, 3196, etc) with different key counts (83 keys, 101 keys, 122 keys, etc) and many different languages.

ATM I have no idea how to make a proper autodiscovery and autoconfiguration for every terminal-keyboard-language combination, so the user will need to configure this editing the 5250_terminal.py script. This is also a matter of personal preference because the older terminals have weird key legends and non-standard layouts, and the user will have to decide the key mappings that better suits his preference.

There is at the beginning of the script a dictionary definition called scancodeDictionaries. That dictionary has one entry for each keyboard mapping available, you have the following mappings available:

Refer to this document for more information about layouts and the scancodes generated.

To change the default mapping used if no mapping is specified in the command line, you have to edit the value of the variable DEFAULT_SCANCODE_DICTIONARY

DEFAULT_SCANCODE_DICTIONARY='5250_ES'

To create a new mapping, you can copy and modify an existing mapping, adding a new entry to the scancodeDictionaries structure and change its name. The entry will look like this:

'YOUR_MAPPING_NAME':
{
'CTRL_PRESS': [0x57],
'CTRL_RELEASE': [0xD7],
'ALT_PRESS': [0x56],
'ALT_RELEASE': [0xD6],
'SHIFT_PRESS': [0x54],
'SHIFT_RELEASE': [0xD4],
'CAPS_LOCK': [0x7E],
'EXTRA': [],
0x7C: [chr(0x1B), chr(0x1B), '', ''],
0x23: ['e', 'E', '', chr(0x05)],

more scancode mappings...

},

In the first entries you have to configure the scancodes that will activate the SHIFT, CONTROL and ALT key modifiers (_PRESS) and those that will deactivate them (_RELEASE). Note that in your keyboard there will be special “break” keys that will generate a scancode when you press the key and another different scancode when you release it. You should use those keys for SHIFT, CONTROL and ALT operation.

So for example if pressing the SHIFT key you generate a 0x54 scancode and releasing it a 0xD4 scancode, you configure the SHIFT mappings like this:

'SHIFT_PRESS': [0x54],
'SHIFT_RELEASE': [0xD4],

If any of these keys are repeated and more than one scancode is available for that function, simply put the extra scancodes inside the brackets separated by commas. For example if both 0x11 and 0X58 scancodes are available for CONTROL:_PRESS you configure them like this:

'CTRL_PRESS': [0x11, 0X58],

Using a regular key is not recommended, but it is supported for CONTROL and ALT operation leaving empty the field for CTRL_RELEASE or ALT_RELEASE. In that case if you press CONTROL or ALT it will remain pressed until you press them again or hit another key.

On the contrary, for CAPS_LOCK operation you have to select a regular key and not a break key.

'CAPS_LOCK': [0x7E],

The rest of the entries are regular scancode-to-character mappings, you need to include one for each available key that you want to use, like this one:

0x23: ['e', 'E', '', chr(0x05)],

Some entries will have 5 fields inside the brackets like this one:

0x63: [chr(0x1B), chr(0x1B), chr(0x1B), '' ,'A'], #up arrow

The fifth entry is an (optional) additional character that is sent when the first character generated is 0x1B (ESC). That’s needed because some keys need to generate escape sequences instead of single characters. In this case the “up arrow” key generates the sequence ESC-A. In the default configuration only the arrow keys need to be configured in this way.

To assist in the keyboard configuration you can run the script with the -k parameter. With this parameter one log line will be added to the debug.log file for every scancode received from the terminal, so that you can know the correct code to use in the mapping configurations.

$ tail -f debug.log
RECEIVED SCANCODE: 0x2d FROM TERMINAL: 0

Once configured, you can make your new entry the default editing the value of DEFAULT_SCANCODE_DICTIONARY or refer to it when running the application like:

$ python3 5250_terminal.py 0:YOUR_MAPPING_NAME

Custom character conversions

When displaying the converted ASCII characters in the 5250 terminal with your selected codepage, you may dislike some of the default character mappings. Even worse, some ASCII characters will be totally missing in EBCDIC and a replacement character has to be selected. For that reason you can add to your keyboard mapping a CUSTOM_CHARACTER_CONVERSIONS entry like this:

#Custom character conversions, from ASCII char to EBCDIC code that will override the DEFAULT_CODEPAGE conversions
'CUSTOM_CHARACTER_CONVERSIONS': {
'[': 0x4A,
']': 0x5A,
'^': 0x95,
'#': 0xBC
},

This will, for example for the third entry map the ^ ASCII character to the 0x95 EBCDIC character (n) for the terminal that uses this keyboard mapping.

Command line interface

When you run the script a 5250> command prompt is presented where you can send some commands to any connected terminal.

$ python3 5250_terminal.py
Searching for terminal address: 0; with scancode dictionary: 5251_ES; slow poll active: False; EBCDIC codepage: cp037

Welcome! Type ? to list commands
5250>

`

Type ‘?’ at the command line for a list of available commands.

So set the active terminal which we will send commands to, type setactiveterminal followed by the terminal number:

5250> setactiveterminal 1

We can for example send a ESC_E command to clear the screen:

5250>escE

All VT52 escape sequences are available to send at this interface

We can also simulate typing a command in the terminal with the input command:

5250>input ls -la

This will “type” the command “ls -la” over the shell of the active terminal like if it had been typed in the terminal.

We can also generate some simulated keyboard scancodes in hexadecimal notation

5250>sendscancode 0xE0

We can restart the terminal shell with the restartterminal command followed by the terminal number:

5250>restartterminal 1

To exit the session and close the program just type exit at the prompt

5250> exit
Bye