The trip point is specd to be between -3 to +3, which is why 5V works,
especially since the trip point is usually positive. I would set it
around 1.5V. The transmitters are required to swing +/- 5V.
I'm not sure the 232 "allowed" lower voltage as much as all we had to
work with were charge pumps, first off a 5V rail and later 3V. Basically
the voltage was dropped closer to the spec limit to ease the design,
even if some non-compliant parts wouldn't work on the lower voltages.
On 12/9/2012 11:25 PM, Hal Murray wrote:
[email protected] said:
I'm not able to get it to respond to the 'S' command and when I measure the
voltage on the RS232 TX pin (#2 from the left) it's always 0v. Shouldn't it
be -12v when idle?
Newer RS-232 allows 6V rather than 12.
In practice, it's not all that uncommon for designers to save a chip and just
send 5V CMOS signals. That works fine for short distances.
If you can get a scope on it, sometimes embedded boxes send out a hello
message at power up.
I'd also check pin 3 in case you or they got things swapped.
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