TTL signal is weaker when connected from CD4069 to RX

I use RS485-TTL interface with buffer CD4069. Like this: SCM TTL to RS-485 Interface Module - ProtoSupplies but from aliexpress. I tried 2 interfaces.

It's not working because arduino receives bad signal on RX. When bypassing buffer directly from MAX485 it works. I checked with oscilloscope that signal directly from MAX485 and from buffer before I connect to arduino is same, but gets weaker and looks distorted when I connect signal from buffer to RX. Any idea why?
Voltage measured with DMM on RX is 2.3V when connected from MAX485 and 3.3V when connected from buffer.

Signal from buffer before I connected to RX (same is directly from MAX485 connected and not connected to RX)

Signal from buffer after I connected to RX

Okay, I have to reconsider my answer... I see now the schematic is actually of the entire module.

Maybe, you are not connecting the module output to the Arduino pin that you think you are. Or, that pin is not configured correctly.

Thus, you should present a complete wiring diagram. It will speed up the troubleshooting process.

This is not a buffer. CD40.. series has typically poor performance with 5V supply. Better use SN74HC.. types with 5V logic.

1 Like

Yes, the CD40xx series is essentially obsolete unless you desperately need to use a higher Vcc than 5 V.

The 4000 series can't source or sink much current (from 5V supplies think microamps, not milliamps) - I suspect the 2k4 resistor and LED are overloading the chip.

Use the 74HC14 as indicated in the schematic, you need the Schmitt trigger on the RX side for clean reception. 74HCxx series can handle several milliamps no problem.

BTW that schematic doesn't show decoupling caps - you still need them, 100nF for each logic chip, very close to the pins.

1 Like

@arti-g, are you prototyping your own circuit, based on the module? Or no?

Thanks for your answers :slight_smile:
CD4069 looked like replacement for 74HC14, it's also hex inverter. No idea why, but someone form China put CD4069 in place of 74HC14. If I understand correctly CD4069 is just hex inverter not buffer while 74HC14 is hex inverter schmitt-trigger and it is buffer. So it must bring that problem I have. I will try to replace it with 74HC14 to check and look for proper modules. I ordered rs485-ttl module from aliexpress because I liked the idea of having MAX485 module easy to connect, with TVS, polyfuses and buffer for cheap.
I'm working on dmx controlled stepper motor.

try this

5 times to replace a single 74HC14? :frowning:

1 for Tx and 1 for Rx

Please note: The 4069 is a CD4069UBE, that is un buffered - it is like the 'A' series CMOS chips. It has low output drive and slow output response. Do not use this chip,

1 Like

And what for the fifth TXD pulse shaper?

The 555 is outdated In the given application by package count, price, performance and power consumption.

Because it was cheaper, or they happened to already have a huge number in stock for some other product.

The terminology is not relevant. The 74HC14 does have a Schmitt trigger input, but so does a CD40106.

And the buffering is not significant either. Since the "buffer" part is an inverter, whether 4000 series chips include this buffer depends entirely on their logic function; whether it is convenient to include that (extra) inverter stage.

The simple fact is that the CD4000 series was the earliest CMOS series using the chip technology of the time. The 74HC series uses a much enhanced design technology which facilitates substantially lower effective internal resistances, thus substantially higher current driving capability and as a consequence, greater speed (because speed in CMOS deices involves the ability to rapidly charge and discharge gate capacitance).

So you simply need to use the later series instead of the obsolete one. :face_with_raised_eyebrow:

And that is pretty much the situation. :grin:

1 Like

Thank you, I replaced CD4069 with 74HC14 and it works, signal is ok!

I saw modules with 74HC04 but since it's not schmitt-trigger it is still better to put 74HC14.

The Schmitt trigger is not strictly necessary, but it should clean up/ keep clean the signal somewhat.

Especially if you mount it near the strip rather than the Arduino.

1 Like