Hoppla, jetzt lese ich gerade Hardwarefehler2.

Was für einen Schnittstellenkonverter hast Du denn angeschlossen?
Original-RIB, Nachbau-RIB, oder RIB-less mit dem MAX232?
Wenn letzteres, dann die Schaltung mit oder ohne 7407?

Bei einigen Geräten gibt es da Probleme mit den Vorwiderständen der LED

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Notes:

If you have problems getting this interface to work on your GP300, then try replacing the two 680 ohm resistors that feed the LED's. If you use 4.7k ohm it should work OK. I found that when using version R01.00.00 software I have no problems with the 680 ohm resistors, but for some reason when I ran the newer software (R08.02.00) I had problems reading the codeplug etc. All I have done is replaced the 680 ohm with 4.7k ohm and it works fine now with both versions of software. You will have to experiment a little and let me know what resistors work for you.

Simon VK4TSC encountered a problem where the GP300 being programmed/read will fail to pass data to/from the computer with an Error #2 (Serial Bus Error). After checking the BUS+ line with a CRO it was found that the GP300 could not properly switch the data bus, this being caused by the led/resistor combination on the data bus drawing a little more current than the radio could supply. By changing the series resistor to 1K ohm instead of 680 ohm the problem was overcome

The better solution maybe to use a spare gate from the 7407 to drive the led separately. Disconnect led cathode and link it to pin 4 of 7407, isolate pin 3 of 7407 from 5V and link it to pin 2 of same chip. The original 680 ohm series resistor can be left as is. BUS+ is now isolated from the data led.
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Quelle: http://www.g1ivg.com/motgp300.htm

Gruß - Karlson