Using C-Kermit with HP Calculators: HP48 (Stéphane Gourichon) HP50g (Frank Singleton)

HP48 / C-Kermit file transfer

Prepare hardware

Gather hardware

• Get or make a serial cable from HP48 to DB9 (most common) or DB25 (instructions on https://www.hpcalc.org/hp48/docs/faq/48faq-12.html, the core of it being: looking at the calculator socket from left to right, pins are shield,tx,rx,ground ).
• Get a USB-to-Serial adapter with matching DB connector, or add an adapter. Beware, some cheap adapters don't respect RS232 levels (-12/+12) and do 0-5V instead, this can be a cause of failure to communicate.

Ensure PC recognizes hardware

We assume here that the PC side operating system runs Linux.

• Plug the USB-to-serial adapters: (PC)[USB]---[USB](USB-to-Serial adapter)
• In most cases, the adapter will be recognized by Linux as /dev/ttyUSB0.
• Running dmesg may provide a confirmation, or hints if the port is different. (Depending on distribution and settings, you might need root access to run dmesg.)
• Plug the rest:
  
  (PC)[USB]---[USB](USB-to-Serial adapter)[DB]---<hp48cable>---[4pin connector](HP48)

Prepare software

Information: why the software choice below

• A ckermit package used to be included in Debian (and thus derivatives like Ubuntu) but no longer. Edit: C-Kermit maintainer is working with Debian (in 2020) to get it included again!
• A gkermit package exists but it seems restricted to a different use case: transferring files on a text-terminal-oriented session, which is what was common before TCP/IP and the web, and does not fit this use case.

Get C-Kermit source my-tt and build it

• From Open-Source Kermit Project, follow link to source my-tt download, currently http://www.kermitproject.org/ck90.html#source
• Some users, especially Ubuntu and Debian users as of 2020, will need C-Kermit "Daily" Source-code Archive that "Fixes a problem in latest Ubuntu and Debian distributions."
• Extract archive, you need basic build tools (compiler, make), compile. If you don't know how to compile software you will have some hints on http://www.kermitproject.org/ck90.html#unixbuild (you need some fairly basic command line practice).
• On my machine it generated a wermit executable

Perform transfer

Run software, set HP48-specific parameters

Since Kermit protocol is very general and this implementation supports many use cases, software needs some parameters to work in our case. The lines below worked for me, words after the semicolons are comments for the curious.

./wermit ; Start C-Kermit ; The following commands are entered at the "C-Kermit>" prompt... set port /dev/ttyUSB0 set speed 9600 ; Serial port speed, HP48 defaults to 9600, also supports 4800 2400 1200 set carrier-watch off ; It's not a modem connection so ignore carrier signal state. set modem type none ; No modem is involved; don't send AT commands. set flow none ; Flow control is not supported by HP48. set parity none ; 8 data bits, no parity set control prefix all ; HP48 Kermit requires all control characters to be encoded in printable form. set file type binary ; Might not always be needed.*

*	Safe default IMHO because it avoids the transfer program to corrupt the transferred data believing we expect it to change line feed characters and the like.

Note that it is not necessary to type all these commands every time you want to set up a file-transfer connection with the HP48. You can copy the commands into a file (e.g. "hp48.ksc") and then tell C-Kermit to "take hp48.ksc". Or you can define a macro in your ~/.kermrc file.

Transfer files

From that on, it's up to your knowledge of the Kermit protocol.

For example to send from PC to HP48, on the HP48 start a kermit server (menu or SERVER command), and at the C-Kermit> prompt type:

send myfilename

Or to send from HP48 to PC, on PC type

server

and on the HP48 put a file name on the stack (with VAR and possibly NXT, find your object in the menu, press ' and the A-F key showing the object) and issue SEND command.

Tested, success

Tested today on a PC running Xubuntu 20.04.

• PC to HP48 with file DM48 from DM48. A Dungeon game for the HP48 , works!.
• HP48 to PC with a simple file containing string "Hello World"

Both worked!

• The original version of this page, which might change over time.
• The Gouri HP48 Collection
• A Dungeon Game for the HP48

Addendum for HP50G from Frank Singleton, May 2022

The following has been observed using C-Kermit 9 on Linux Mint 20.3, when transferring files to a HP50G calculator using USB connector.

Sending files to the HP50G using C-Kermit, may result in NAKs being sent from the calculator (receiver) back to C-Kermit (sender). It was observed that small files with data packets around 50 bytes were ok, but larger ones (approx 100 byte data packets) would fail with "Protocol Error" being displayed on the calculator.

According to section D-4 of "HP 50g / 49g+ / 48gII graphing calculator advanced user's reference manual", it mentions:

receive pacing

Controls whether receive pacing is used: a nonzero real value enables pacing, while zero disables it. Receive pacing sends an XOFF signal when the receive buffer is almost full, and sends an XON signal when it can take more data again. Pacing is not used for Kermit I/O, but is used for other serial I/O transfers.

transmit pacing

Controls whether transmit pacing is used: a nonzero real value enables pacing, while zero disables it. Transmit pacing stops transmission upon receipt of XOFF, and resumes transmission upon receipt of XON. Pacing is not used for Kermit I/O, but is used for other serial I/O transfers.

So it appears HP50G Does not support XON/XOFF for kermit I/O.

Via some testing, and consultation with the C-Kermit maintainer, it was suggested that the HP50G may have an incoming packet length limit but does not tell the file sender (C-Kermit in this case) about it in the negotiation phase.

To avoid filling the small receive buffer of the HP50G, resulting in repeated NAKs and Protocol Errors, we can tell C-Kermit (sender) to send smaller packets.

Adding the following line to ~/.kermrc appears to fix the issue.

set send packet-length 80

[Ed. - If C-Kermit is to be used for other applications besides this one, the "set send packet-length 80" command would go in any scripts written for HP50G interaction, rather than in the ~/.kermrc file.]

Reference: hp50gshell: Use Python to expose a simple interactive shell for HP 50G calculator.

Last update: Tue May 24 07:08:53 2022

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