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I am trying to make the GPIO serial port of a Raspberry Pi 2 with Raspbian Wheezy work with an elevator (to control its movement and check status). But before I even plug it into the elevator, I am testing whether I am sending signals correctly from it. As I was having difficulty, I thought I would first test to see if at least my hardware setup is correct, so I reinstalled Raspbian so that the serial console would be re-enabled. I figure that if I can at least get the serial console up and running, I am not wrong on hardware.

The board I'm using is this one:

RS232 adapter board

I have a too-low reputation to post the link to the board vendor or the instructions I'm following.

I'm connecting TX-to-TX and RX-to-RX like this:

Connection to GPIO

I'm plugging it into a PC serial port directly (I found that using a 3m cable made it return garbage to minicom, so is the power too low or something?).

I'm using minicom over /dev/ttyS0 with 115200 8N1 settings, no hardware or software flow control.

When I turn on the Pi by plugging it in with minicom running, I get the 'T1' LED flashing on the serial adapter card, presumably in time with the boot sequence. By itself, nothing appears on the minicom screen.

During this bootup if I hit a key on minicom, the R3 light turns on, and stays on permanently. However, I see the remaining bootup sequence, and it returns to a login prompt. But I cannot type anything to log in. It responds to no keyboard input at all. I have a feeling it has something to do with the R3 light being on permanently. I am doing something right, because a single keyboard tap works to force the Pi to send or the PC to receive the bootup stuff.

What could I possibly be doing wrong here? Is it a minicom setting or am I using the MAX3232 converter wrong?

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Unless you have an oddly labelled board you should be connecting board TX to Pi RX and board RX to Pi Tx.

Do not connect the VCC line unless you are powering the Pi from the serial board (unlikely). The two 3V3 power supplys (Pi and the board) will be fighting.

  • I've heard very conflicting instructions on this. If I'm doing the wiring backwards, why is minicom receiving the bootup text? – Asad Sayeed Aug 28 '15 at 15:37
  • And from where would the serial board be getting its power? The PC? – Asad Sayeed Aug 28 '15 at 15:38
  • I didn't read carefully enough. I missed the bit about hitting a key in minicom. Is /dev/ttyS0 the PC end? You need to measure the voltage at the PC end on VCC. If it's not 3V3 you will be damaging the Pi. – joan Aug 28 '15 at 15:44
  • Yes, /dev/ttyS0 is the PC end. I (ignorantly) thought the whole point of the serial card was to protect it from the PC. Most of the online instructions don't give anything else required to plug into the PC. What should I use if I want to establish a connection between the Pi and something that uses PC power levels? – Asad Sayeed Aug 28 '15 at 15:56
  • I'm sure the card is right for what you want to do. The problem is knowing what the board is actually doing. The MAX3232 chip is well documented but that says nothing about how it operates when installed in an interface board. Could you post the link to the board specs? Just remove the front http or whatever so that it's just plain text. – joan Aug 28 '15 at 16:03

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