Controlling HDMI Switch via RS232/UART

Projects Programming
1

Problem:

I have an HDMI switch that is controllable via RS232

Scenario 1:

Setup: PC connected to HDMI switch using USB -> serial adapter
Test: Opening PuTTY on COM port and sending command "sw i01" from my PC
Results: HDMI switch input changes to port 1 [Good]

Scenario 2:

Setup: Arduino UART port connected to TTL->RS232 adapter connected to serial->USB adapter connected to PC with PuTTY program open on COM port
Test: Sending command "sw i01" from Arduino to PC using code below
Results: "sw i01" prints repeatedly on the Putty terminal [Good]

Scenario 3:

Setup: Arduino UART port connected to TTL->RS232 adapter connected to HDMI switch serial port
Test: Sending "sw i01" from Arduino repeatedly to HDMI switch using code below
Results: HDMI port does not switch from port2 to port1 [Bad]

Hardware:

Intel Galileo Gen 2
SparkFunRS232/TTL Shifter connected to Galileo UART
HDMI switch is IOGear GHSW8141
Code:

[Note: Galileo Gen 2 requires Serial1 for UART communication]

void setup() {
Serial1.begin(19200);
}

void loop() {
Serial1.print("sw i01");
Serial1.write(13); // CR
Serial1.write(10); // LF
delay(1000);
}

Question:

It seems like Scenario 1 and 2 confirm that the Arduino->HDMI switch communication should work, but it doesn't. Am I missing something regarding physical connectivity? Does manually typing "sw i01" in PuTTY send different commands than I have in the Arduino code?

2

I found it is was related to the physical connection!

In Scenario 3, I originally had the Sparkfun TTL/RS232 adapter that had a female DB9 connector wired to the HDMI switch female DB9 connector using a male/male serial cable. The serial cable I bought was a straight through cable.

I swapped it out with a null modem cable and it worked right away.

Looking at the wiring diagram, I thought swapping the Tx/Rx pins on the Sparkfun adapter would have achieved the same result as the null modem cable, but apparently not.

If anybody can point out why the null modem cable made the difference, that would be something good to know for the future.

3

I don't remember all the signals off hand (you could search for null modem cable and find the differences), but often CTS, DCD etc. are also involved - TX and RX are the data lines, but the others are often used for flow control (CTS is Clear To Send, RTS is Request To Send etc. - there are basically 2 types of flow control (3 if you consider "none" as a type) - hardware which uses the CTS, RTS, DCD etc and the software type with the control characters XON and XOFF being sent back to control the data flow). I'm so old, I still remember thinking how fast 300 baud modems were !! :o