SCI1 TX problem

[slow]

I am able to receive characters on the SCI1 RS-232 port just fine, but have not found a recipe that lets me transmit successfully. I tried connecting SCI1 (through a null modem) to a serial port on a second computer with the same results. My cable connects pins 5, 7, and 9 on J4 to a DB-9 pins 1, 2, and 3.

My cable tests fine with a multimeter. I made the connector with the crimp tool in the crimp kit and it looks fine.

J4 pin 9 (SOUT1) has -5.41 volts relative to J4 pin 5 (GND) with or without my device connected. J4 pin 7 (SIN1) is 0 volts with no device connected and -8.13 volts when I connect my cable. I don't know if these are in an expected range. For comparison my Dell's serial port has 11.04 volts on pin 2 and -10.77 volts on pin 3 relative to pin 1 ground.

Am I missing something simple? Is it possible I have a defective SCI1 port?

Thank you,

slow

[nmitech]

The serial cable should wire like this,

---J4----------------- Straight thru ----- Null Modem
Pin 5 (GND) <-------> Pin 5 (GND)------- Pin 5 (GND)
Pin 7 (SIN1) <------> Pin 3 (TxD)-------- Pin 2 (TxD)
Pin 9 (SOUT1) <----> Pin 2 (RxD)------- Pin 3 (RxD)

[RMDumse]

Originally posted by slow
J4 pin 9 (SOUT1) has -5.41 volts relative to J4 pin 5 (GND) with or without my device connected. J4 pin 7 (SIN1) is 0 volts with no device connected and -8.13 volts when I connect my cable. I don't know if these are in an expected range. For comparison my Dell's serial port has 11.04 volts on pin 2 and -10.77 volts on pin 3 relative to pin 1 ground.

Those are in the expected range. RS-232C spec requires a transmitted "mark" or logical one to be -3 to -15V, and a "space" or logical zero to be +3 to +15V.

Generally your serial out pin will be some negative voltage (lower than -3). Your serial in pin will be ~0 V with nothing hooked up, because it is a recieving input, and shouldn't be generating anything. That's the transmitting signals job coming from the other side.

What program are you using to see the characters? Most likely you are missing something about the program or wrapping handshaking signals back around so the other unit sees DTR/DSR or CTS/RTS. Usually, these signals are just shorted together. If handshaking is enabled, and not present, these signals will not allow characters to be recognized.