This is just a blog regarding the inherent flaw with the pinout of the download connector when used with a standard (i.e. proper) serial port or a RS232 USB-to-Serial adapter.
This is what the download circuit looks like when connected to a PICAXE.
With the plug disconnected, if you were to measure the voltage between the tip and the ring, the ring will be 10V less than the tip. The low current can be clamped to 0 by the Serial In pin when this is connected.
When the plug is partially disconnected, the connections are mixed up as shown. Because Serial Out is a low output, it behaves like the proper 0V connection on the download connector. As the proper 0V on the download plug (the tip) is no longer connected to the proper 0V of the download connector, the 10V on the ring becomes the ground level of the plug. Now, the tip on the plug is 10V more than the ring. As a result, whichever connection in the download socket is connected to the tip gains 10V potential in relation to the circuit's ground (0V) and that connection happens to be Serial In. The Serial In pin on the PICAXE clamps this to 5V which is high, so the PICAXE thinks a program download is being started and tries to accept data that never comes. After a short delay, the PICAXE resets.
Workarounds for the issue, if it is an issue for you:
This is what the download circuit looks like when connected to a PICAXE.
With the plug disconnected, if you were to measure the voltage between the tip and the ring, the ring will be 10V less than the tip. The low current can be clamped to 0 by the Serial In pin when this is connected.
When the plug is partially disconnected, the connections are mixed up as shown. Because Serial Out is a low output, it behaves like the proper 0V connection on the download connector. As the proper 0V on the download plug (the tip) is no longer connected to the proper 0V of the download connector, the 10V on the ring becomes the ground level of the plug. Now, the tip on the plug is 10V more than the ring. As a result, whichever connection in the download socket is connected to the tip gains 10V potential in relation to the circuit's ground (0V) and that connection happens to be Serial In. The Serial In pin on the PICAXE clamps this to 5V which is high, so the PICAXE thinks a program download is being started and tries to accept data that never comes. After a short delay, the PICAXE resets.
Workarounds for the issue, if it is an issue for you:
- Add a PTB switch on the serial in connection which is to be pressed when plugging in/unplugging the cable
- Switch to the 3-pin molex connector
- Shell out for the AXE027
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