[uClinux-dev] Serial Port woes

Zhu, Kunlun kzhu at newportinc.com
Wed Mar 26 12:38:55 EST 2003

Some hardware drivers for RS-232 can be in auto-detective mode which means
only the input line is RS-232 voltage level(CTS for modem side), the
output(RTS)can work. That's why the loopback didn't make it work. Check the
voltage of RTS and DSR on modem side, it should be 0 (Normally it should be
-3~-12v for logic 0, or +3~+12v for logic 1).

Hope this help.


-----Original Message-----
From: Daniel Ng [mailto:danieljng at yahoo.com]
Sent: Tuesday, March 25, 2003 11:31 PM
To: uclinux-dev at uclinux.org
Subject: [uClinux-dev] Serial Port woes


I'm trying to communicate with a 3-pin RS232-based (only Tx, Rx and
Common pins on the connector) serial device via the 9-pin RS232 port on
my uClinux-based product (SnapGear SE1100).

So far, I can connect the serial port on the SE1100 to a modem, and
successfully communicate over that connection (via the 'tip' console

However, when I plug the 3-pin serial device into the SE1100, I can't
seem to be able to talk to the serial device at all (absolutely no
response from the device). If I plug that device into a PC, I have no
problems with talking to it.

I've tried- 

1) Mucking around with the stty command(in particular setting clocal
and -crtscts)

2) Shorting the DSR and DTR pins together, and having a (separate)
short between the RTS and CTS pins 

3) Using different combinations of speed and different types of flow
control, spacing, etc.

4) Putting a RS-232 tester box in series with the connection. LEDs on
the box show which pins are active.

1,2,3 don't result in any difference in behaviour (ie. no response from
the 3-pin device).

Using 4, I found that when I connected the SE1100 to the modem, the CTS
and DSR pins are active, but when I connect the SE1100 to the 3-pin
device, none of the pins are active.

Any ideas most welcome...


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