Fanout and RS232/TTL converter

Thread Starter

sonnichs

Joined Mar 29, 2024
2
I have a microprocessor simultaneously delivering 3.3V serial signals to a PIC processor and a MAX3232 converter. I would think this is acceptable - a fanout of at least 10 is allowed for CMOS and TTL. The MAX3232 or the PIC serial code work fine individually---but I find that when I connect both at once I do not see signals from the MP.
Has anyone experienced this? Seems like my design is OK here.

thanks
Fritz
 

AnalogKid

Joined Aug 1, 2013
11,196
a fanout of at least 10 is allowed for CMOS and TTL.
Not exactly. A standard CD4000 series CMOS output running on 5 V cannot drive 10 TTL inputs. Also, a heavily loaded TTL output might not produce a high enough voltage to be seen by CMOS as a logic 1.

And, paraphrasing Rear Admiral Joshua Painter,

"Engineers don't take a dump, son, without a schematic."


ak
 

Thread Starter

sonnichs

Joined Mar 29, 2024
2
Don't think this requires a schematic as it is really a generic question about the fanout. I probably have one in Altium around here somewhere. If I can find where long gone predecessors left it I will send it on.

I know that some breeds of ICs will load inputs more than others but I find it odd that we see this with only two here-- a MP and the MAX3232. The MAX3232 inputs are in the nano-amps and although I don't have the stats for the processor (TX2) it would be surprising to create that type of draw. We are at 9600 baud here and I don't expect pF pins to cause a problem.
But forgoing all this-have you seen this-I have generally assumed with most modern ICs I can spread an a "TTL" output from a MP into 2 devices. I have done it before and did not think I was stretching the rules.

Your humor is appreciated--I am neither and engineer (I am a physicist) nor a Russian (I am in the US). That said I do work much better from schematics when they are around.

thanks
fs
 

MrChips

Joined Oct 2, 2009
31,076
I beg to disagree. A circuit schematic is the language in electronics. If you want to understand the behaviour of the circuit the first step is to study the schematic.
 

sagor

Joined Mar 10, 2019
927
I suspect one device is loading the other. To share a common "signal line" or "buss", designs usually use an open collector design. Signals are held high and whichever device wants to use the lines, draws the signal low. This was (is) common in computer circuits where more than one device is attached to a data bus or address lines.
Sending out with a fan-out may work in general, but the receiving lines (input) may conflict as one may have pullups internally while another does not or has low pin impedance.
If both devices idle at a high state for the RX lines, then putting a small pull-up resistor to Vcc on the line may help make it work. If the RX lines idle at a low state, then one device going high when active may be pulled down by the other device, and it will not work together.
 

Ian0

Joined Aug 7, 2020
10,269
It could be that the PIC pin is configured as an output.
It could be that the grounds of the two circuits are not connected together.
It could be that the other circuits are on different supply voltages
Without a circuit diagram it's a complete guess.
 
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