expandability

LucidGuppy

What are peoples experience with making an expandable system.

Do people prefer edge card connectors like PCI?
Do people like dual row headers mounted vertically?

Just trying to get people's thoughts.

Regards,
Lucidguppy

Mike Green

It depends on what kind of expandability you're talking about. Edge card connectors are useful at very high speeds with a lot of parallel signals. PCI has a full width 32 bit bus with separate data and address lines. On the other hand, manufacturers are getting away from parallel busses, particularly for storage, partly because of the cost of the connectors and the board area to carry the busses. Often, the peripheral devices that were on the PCI bus are now integrated into the CPU or a second package that's next to it on the board. Similarly, USB, Firewire, and Serial ATA have eliminated external parallel connections. For something on the scale of the Propeller, headers are not a bad idea. The convention of a row of servo headers isn't bad either. You can use a block of 3 pin female headers to get power, ground, and a few (2-6) signals, or you can use a single row of female headers to get the signals and a separate 2 pin header to pick up power and ground (to reduce cost and insertion force). If you're using I2C, a pair of 3 pin headers will get you the two signals, power, and ground.

Problems with headers ... They're not normally keyed although you can get specific sizes of keyed headers. If you want a general purpose expansion header, I'd suggest two keyed 20-pin dual row headers, one for the first 16 I/O pins, and the other for the second 16 I/O pins. The extra 4 pins can be used for a ground, Vin, +3.3V, and +5V. I wouldn't want more insertion force than that and you rarely would use that many I/O pins for any particular application. You can always use both connectors for a project that needs it.