Programming Circuit Design Queries

[Clive Wallis]

Hi,

I've always been curious about the design of the simple programming circuit which is used with PICAXEs. Are the resistors there for protection of the PICAXE, or do they perform some other function?

I'm in no way critical of the design, as it has always worked perfectly for me. In fact much better than some of the designs for other PICs, which I've found to be very critical with circuit design & RS232 lead length.

Another question. Is there any reason why the body of the programming jack is used for one ofthe signals? Again just curious, no proglems todate.

Cheers

Clive

 

[Technical]

The first question is answered under the 'serial download circuit' section of manual part 1. The 22k is for protection, the 10k to stop floating signals when the cable is removed.

Cable layout is just historical. This was the layout used with many early (serial, not USB) digital cameras, so this was the layout chosen as these camera cable were readily available at very low cost at the time.

 

[womai]

Unfortunately, that conductor assignment creates problems when you try to use a metal enclosure. Suddenly a data line is connected to the chassis/enclosure, and that won't work if the chassis is grounded (e.g. because there is also a power connector which has it's shield connected to ground) or whenever you touch the box with your hand. Been there, done that, and it took me a bit to figure out why one of my designs was malfunctioning intermittently while the board had worked flawlessly without enclosure. I'm now using standard DB-9 connectors and cables in those situations, but that prevents me from using the nice and compact USB download cable - instead have to use the clunky USB-to-serial converter.

 

[Clive Wallis]

Hi,

Many thanks for the explanation. Much as I thought re the circuit. Interesting about the digital camera leads.

Regards

Clive