MAX232 does not work due to 74HC4052 multiplexer!

[m_jabarouti]

Hi,
74HC4052 or HCF4052 is used as a multiplexer for extending serial ports!!! the outputs of 4052 come to 232 ICs (ST232C, MAX232, MAX3232)! The schematic is here:



The output (green) and input (blue) of 232 ICs are shown in the picture below:



As it can be seen, the output is between -1 and 1! when i remove 4052 and connect the line with wire, everything will be correct! Does it need a buffer before 232 IC?

Thanks for your helps.

 

[betwixt]

We need more information, specifically, are you connecting the 4052 on the RS232 side of the MAX232 or the TTL side?

On the RS232 (V24) side the polarity SHOULD reverse and with a single supply it would be normal for the voltage to be clipped when negative. If that's what you want to do the 4052 will not be rated to hande the full 24V (+12 to -12) swing, even if run on dual supplies.

Brian.

 

[m_jabarouti]

Thank you for your reply!

The schematic shows that the connecting the 4052 on the TTL side of the MAX232!!!
TXD3 from 4052 goes to T1N!!!

 

[FvM]

The circuit is basically O.K. The waveforms don't give much information because no voltage levels are shown.

 

[betwixt]

Sorry! I'm 'homeless' at the moment while building works are undertaken and all my lab equipment and computers are packed away for the next few months so I'm reliant on mobile devices. It's difficult to read diagrams on a 12cm smart phone screen!

FvM is quite right but answering your original question, no buffer should be necessary.

Brian.

 

[spudboy488]

One thing I notice is that the charge pump cap connected to Vdd (Pin 2) should go to ground and not 3.3V. Filter caps C311 and C312, if they are truly polarized, are in backwards.

 

[FvM]

One thing I notice is that the charge pump cap connected to Vdd (Pin 2) should go to ground and not 3.3V.

Should it? You'll find different connection in different datasheets. I suppose both will work.

 

[D.A.(Tony)Stewart]

I agree CS11 and CS12 are reversed on schematic.
RS232 are terminated with 10K type loads and use 1.3V as the receiver threshold, identical to TTL.

V+ and V- pins 2,6 ought to show the bipolar supply charge voltages correctly, when everything is correct polarity. As it is current limited to 60mA a reversed cap can cause malfunction to V+.

The Cap C10 on pin 2 V+ ( your Vdd) may also be reverse biased under transient power on off condition. This may be best to reverse it as shown so cap polarity connected to low impedance power supply (3V) is same polarity as polarity of connection.. i.e. if cap from pin 2(+) to supply then use + lead to 3V supply and if cap connected to ground, use cap(+) side to pin 2V+ and (-) cap to ground.