Serial Out's connected together

[SteveT]

I'm designing a 'M2 parts' project board for my own use and just wondered if I was safe connecting all the Serial Out pins together or would I be better off using jumpers as in the I2C bus of my attached schematic?

Steve

 

[Dippy]

I haven't looked in detail but it's a bad idea to do that. Consider one Serial out going high and another goes low. At the very minimum it will attentuate the signal to nearly nothing, at the worst it could cause damage.
If you have control of the I/O mode you can sniff then change I/O , send, change I/O back to input. It's commonly used on a well-known competitor.

The usual options are:
- Diode mixing and a pullup.
- Using one or two transistors to make each node an open collector/drain (like I2C) and a pullup. One tranny is cheaper but will invert.

In any event you may still have a data clash but at least there won't be any chance of physical damage.
There are other methods but these tend to be easiest and cheapest.

 

[SteveT]

Cheers Dippy, all taken on board. I'll probably go with the diode/resistor pull up's.

Bat85 and 4K7 about right?

 

[SteveT]

Updated schematic (if correct)

 

[hippy]

You'll want pull-downs for that usage. In fact, you can lose the 4K7 pullups on each Serial Out and replace with a single pull-down to 0V on from pin 1 on SJ1.

 

[SteveT]

OK Hippy, will change the schematic. Cheers mate.

 

[SteveT]

Here it is.....