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Scope Output

Hi - I'm getting unexpected output from a MAX3232 board. I'm trying to reimplement a baseball scoreboard controller. The signal I'm trying to recreate is at RS232 levels (-5 to 5V) but uses a different encoding.

I've got a Raspberry Pi Zero driving a 3V GPIO (the blue trace), and the output is hooked up to a little cheap MAX3232 board with a 9 pin serial connector (the signal goes to the DIN1 pin on the 3232). I'm getting the yellow trace output on pin3 (TX) of the RS-232 connector. What I'm expecting is a -5 to 5V copy of the blue trace. What I'm getting looks like the inverse of the first derivative of the signal (low on rising edge, high on falling edge) ? (the range is correct though).

Any ideas what might be happening here? I've heard of MAX3232 knockoffs...but differentiator output?? Thanks for your time!

Update - here's a picture of MAX3232 board: MAX Board

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    \$\begingroup\$ 1. is this AC or DC coupling on your scope? $$\,$$ 2. if it is DC coupling, you've got a capacitor between your transmitter and your scope. Sometimes, a broken cable can be such. \$\endgroup\$ Oct 23 '20 at 17:42
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    \$\begingroup\$ btw, your scope has a screenshot functionality. Direct exports are always desirable over angled smartphone snapshots. \$\endgroup\$ Oct 23 '20 at 17:43
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    \$\begingroup\$ Is the max3232 charge pump pumping? You should be able to pick up +5V and -5V on corresponding pins. \$\endgroup\$
    – bobflux
    Oct 23 '20 at 18:04
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    \$\begingroup\$ @MarcusMüller it is DC coupling \$\endgroup\$
    – evm
    Oct 23 '20 at 19:50
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    \$\begingroup\$ What if you swap blue and yellow probes? RPI or MAX3232 may be damaged already if you used 5V supply for MAX3232, as the RPI uses 3.3V IO levels. Also the RS232 output would not be direct copy but inverted copy of TTL signal. The MAX3232 may also be misused, if the unused TTL inputs are left floating, they should not float. \$\endgroup\$
    – Justme
    Oct 23 '20 at 22:03
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So this turned out to be just a MAX3232 artifact combined with a DB9 pinout error (for which I feel silly). On DB9 the TX pin (2) is supposed to go to the RX pin (3) on the PC side (male connector) - however if you're hooking a male connector up to a female connector you still want to measure the corresponding pin 2. So I was actually measuring the RX pin. When I measured pin 2 I got the -5V/5V inverted clone of the 3V signal I was expecting. So the signal pictured is presumably a spurious signal that the MAX puts on the RX line if it's not connected to anything. I wasn't expecting that, so I got fixated on correcting the output, but really I was just measuring the wrong pin.

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