pull ups/down's on serial lines between IC's

[BrendanP]

I have 2 IC's in serial communication on a pcb design I'm working on.

I think I need to have a approximately 4.7K pull down R on each track/line if I remember correctly? ie. the lines can't be left floating

I looked back at some old pcb designs on previous but can't see them, I seem to remember I had to solder a couple on.

 

[Dippy]

What are the ICs mate?

If they are push-pull logic style then nothing else is usually required.

If they are Open-whatever style then you'll need to know the details.
And the value can often be related to comms speed.

I can't quite see over your shoulder at the old PCB designs... I'm stretching , but no... your ears are in the way ...

 

[BrendanP]

Sorry, I'll pull my ears in mate. Its a 40X2 and a GSM module. Hows your view now?

I had two 40X1's on a previous project and I seem to have a strong memory of Hippy telling me to use two pull downs or pull ups depending in using true or inverted serial coms.

 

[BeanieBots]

For 2 off 40X1's talking to each other, no need for pull up/down resistors but a series resisitor (1k) is a good idea incase of software 'accidents'.

As for the GSM Module, depends on what Dippy described.
Probably NOT required but if it's open collector, then they WILL be required.

 

[BrendanP]

Thanks BB and Dippy, I'll make inquiries re. the module.

 

[Dippy]

That would be a cunning plan. Data Sheets are the first port of call.

I just looked at 5 different Data Sheets.
The main comms for a Micro were UART and SPI.
Normally no pull-up/downs required.

But who knows....? Give us a link. Give us a make. Give us a clue.

 

[hippy]

The need for pull-up or pull-downs on serial connections are when the two ends are disconnected from each other, or one of the two is removed from circuit. This will keep the lines at their idle states rather than floating and receiving garbage. Use a pull-down if using Nxxxx baud rate, pull-up if Txxxx baud rate. The pull-up / pull-down goes on the serial input of each.

 

[inglewoodpete]

A pullup resistor would be required if your output is open collector. This is sometimes used with many-to-one serial connections. My guess is that it's not required in your case.

 

[BrendanP]

I know the modules data sheets very well, unfortunately the module maker is in China and their customer service ethic needs work as does the detail provided in their data sheets. Motorola modules have far better support but don't support offer video txing.

(But then given China is now in effect Americas banker ,so says Hillary anyway, and they are just about to launch their first nuclear powered and no doubt armed carrier maybe they don't really give a stuff what westerners think ,do or say).

 

[Dippy]

Well, if you are designing the PCB why not include a few pads for a pull-up/down just in case...?
You can do that after emailing the Manufacturer and asking them.
Or you could use a proper GSM unit and avoid paying for part of this new Aircraft carrier.

 

[BrendanP]

Given that the Chinese now buy billions$ in iron ore, coking coal, etc. from Australia I don't have any qualms spending $30 on their modules. We'd be stuffed without them. No sense backing yesterdays hero, the US's days of supremacy in the western pacific are numbered in my view.

(I would of used Motorola, made in Israel btw, but their modules don't support 3G video).

The idea of the vacant spare pads had also occurred to me but the pcb is all smd, also being made in China. Got to do my bit helping world trade. I can probably hand solder a couple in if need be onto the mcu pins. I will probably take a gamble with the protos and see what happens.

 

[Andrew Cowan]

If you wanted to take a gamble, I'd go with a 10K pull up (which is extremely unlikely to damage anything), and make the track connecting the resistor to the module (or to the V+ line) very narrow. That is so you can easily cut it if it turn out it is causing something to not work.

 

[BrendanP]

Thanks Andrew, best wishes for the coming year.