Solved Serial TTL help

Phishfry

Phishfry

I am having a hard time understanding serial comms.
Researching using the PC Engines APU3 COM2 port and it uses UART TTL signaling.
I have it working with a MAX3232-DB9 Level converter PCB being powered by 3.3V.
The GPIO pins can also be used for COM ports and they offer way more pins. 8 pins versus 4 pins.
So COM2 UART=TX/RX/Power/Ground for 4 pins.(Just like Arm boards)
But GPIO header uses what appears to be the full run.
###COM4 Serial UART Pins###
pin11=CTS4
pin12=DSR4
pin13=RTS4
pin14=DTR4
pin15=RXD4
pin16=TXD4
pin17=DCD4
pin18=RI4

I have used the TX/RX pair from GPIO and it works fine for a console. I didn't do any speed testing.

So What is the difference here. Can I get way faster coms with 8 wires versus 4? I know RI is Ring Indicator from the modem days.
Obviously this feature is not needed, do these lines just go to waste?

The GPIO serial port lines have to feed into a TTL Level converter too and all those only have 4 pins with RX/TX/Power/Neg .
So is that all it really needs? TX/RX, All the rest of these serial pins go unused?
APU-serial.png
 
Phishfry said:
So COM2 UART=TX/RX/Power/Ground for 4 pins.(Just like Arm boards)
Click to expand...

You don't normally connect the power pin (at least on the rpis and other PIC microcontroller boards) lest you end up backfeeding power and generally bypassing any fuse/regulation circuit on the real power input.

###COM4 Serial UART Pins###
pin11=CTS4
pin12=DSR4
pin13=RTS4
pin14=DTR4
pin15=RXD4
pin16=TXD4
pin17=DCD4
pin18=RI4

So What is the difference here. Can I get way faster coms with 8 wires versus 4? I know RI is Ring Indicator
from the modem days.
Click to expand...

No you cannot get faster comms. CTS clear to send; DSR data set ready; RTS request to send; DCD data carrier detect ... oh the nostalgia for the days of dialup :) So they're a mix of handshaking and connection detection.

So is that all it really needs? TX/RX, All the rest of these serial pins go unused?
Click to expand...

You generally need to also use a ground pin.
 
trev said:
You don't normally connect the power pin
Click to expand...
You are absolutely right there. I mis-spoke. On Arm UARTS I only use RX/TX/ground.

I was messing with multiple COM ports on BBB and had these onhand already -MAX3232 TTL to DB9 adapters.
They require RX/TX/power/ground. They will run on either 3.3V or 5V power input.
https://www.ebay.com/itm/112352426544

The MAX3232 seem to offer the same specs as the MAX3243 built on the APU2/3 except for power control.
 
For async serial comms sending binary data, I strongly recommend hardware flow control if the port supports it. People take way too many shortcuts and it usually doesn't bite them, but instead the guy that comes after them.
 
I guess I could build a custom level converter for more pins.
Right now my db9 converter limits me.
I have the official cable coming soon. I am just antsy.
I also thinks it's pretty cool to have multiple COM consoles from the same box.
 
I did notice on the above schematic that COM2 has RTS2 and its tied to TP5.
What is that about?
The same for DTR2 tied to TP6
 
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