19 Aug
2015
19 Aug
'15
8:40 p.m.
I think the main issue is that TTL is usually a weak pullup and a hard
pulldown.
RS 232 levels are equal up and down.
Putting it on the RS232 lines could even improve signal quality because
the signal is usually poorly terminated, causing reflections.
The recommended lengths and Baud rates are intended to account for this.
Usually the line is lower impedance than the input termination,
requiring the source to absorb the reflected signal.
This is why 422/485 came to be. The impedance of the loads better
match that of the termination.
Dwight
Subject: Re: RS-232 Tx / Rx monitoring LEDs?
From: nf6x at nf6x.net
Date: Tue, 18 Aug 2015 21:27:19 -0700
To: cctalk at classiccmp.org
Those little RS232 testers with LEDs built into a double DB25 connector box are usually
just made of LEDs and resistors connected to each signal line. They can load signals
enough to cause problems at high speeds, with weak drivers, or with long cables, but
usually they don't cause problems. If you're concerned, you could always include
jumpers or switches to disconnect the LEDs when they're not needed.
Of course, buffering the TTL signals eliminate any such problems. But on the other hand,
using two LEDs connected with opposite polarities on each RS232 level signal lets your
discriminate between driven positive, driven negative, and open. That can come in handy
when debugging things where the other end may or may not be driving properly, or may be
mis-wired.
For an example, feel free to take a look at this little modular jack RS232 tester that I
made:
https://github.com/NF6X/YostTester/blob/master/YostTester.pdf
The red/green LED pairs show whether each line is high, low or open. Resistor values may
vary depending on the LEDs that you choose.
--
Mark J. Blair, NF6X <nf6x at nf6x.net>
http://www.nf6x.net/