For something that "there's no such thing as", there sure are a lot of
various null-modem adapters and cables out there.
BTW, while we're picking nits, USB/is/ a serial protocol, so a
USB<>RS-232 adapter /does/ have two serial ports!
As the name suggests, a null-modem cable/adapter/whatever /replaces
a serial modem-to-modem/ connection between two computers regardless
of the protocol used; RS-232 and RS-232 modems have been obsolete for
decades, so the term 'crossover' has become more popular, but it's the
same thing: a cable or adapter to directly connect two computers together.
A pair of modems would connect the transmit signal (pin 2 on an M100)
on one end to the receive pin (pin 3) on the other end. When you
replace those 2 modems (RS-232, USB, whatever) with a null-modem cable
you emulate those modems and do the same thing, connect transmit to
receive etc. (instead of straight-through, connecting transmit to
transmit etc.)
Good luck in your quest to eliminate the term "null modem" in
reference to USB cables; I suspect it'll be around for a while and
more easily understood by many folks than "Provides a DB25M wired as DCE"
FWIW, here's a USB-RS-232 Null modem cable; better send them an email
to let them know that "there's no such thing" ;-) :
https://www.startech.com/ca/Cards-Adapters/Serial-Cards-Adapters/USB-to-Null-Modem-RS232-DB9-Serial-Adapter-Cable-DCE-FTDI~ICUSB232FTN
Here's an example of a USB-USB null-modem RS-232 replacement; DTE and
DCE have no meaning here whatsoever.
https://www.ftdichip.com/Support/Documents/DataSheets/Cables/DS_USBNMC.pdf
And here's an Ethernet null-modem (crossover) cable:
http://www.nullmodem.com/10BaseT-Crossover.htm
m
---- Original Message -----
*From:* Brian White <mailto:[email protected]>
*To:* [email protected] <mailto:[email protected]>
*Sent:* Friday, May 31, 2019 5:56 AM
*Subject:* Re: [M100] possibly ideal serial cable?
Technicality:
There's really no such thing as null-modem in a usb adapter.
Null-modem vs straight through describes the wiring between two
serial ports. A usb adapter only has one serial port.
That single serial port can be wired to be either a DCE port or a
DTE port. Most computers or host devices (like terminals) have DTE
ports, and most peripherals (like modems) have DCE ports, so that
a modem can connect to a terminal with a simple straight-through
cable.
Most usb adapters are meant to drop in place of the serial ports
that computers no longer have, and so they provide a DTE port.
A custom non-standard one could provide a DCE port instead, and
could be made even more non-standard by having a DCE port with a
female connector.
So, it's not null-modem, but it is "Provides a DB25M wired as DCE"
------
Opinion:
I personally wouldn't like to combine the serial cable and the usb
adapter in one, especially not if the serial port will be so
unusual. I don't like uneccessary adapters and connection points,
but I do like standard interfaces. So there is a tug of war or a
balance between those two aims. I like the usb part to be
generically useful on it's own with anything else, and if the usb
part breaks, you can swap in any other standard usb adapter. Same
for the serial cable. It doesn't have a chip that could go poof,
but a wire or a pin could just break. If it's a standard cable, it
can be replaced with any other standard cable.
I also don't like adapters hanging off of device ports directly.
I'd rather have the rigid plug that sticks out of a usb port or a
serial port be as small as possible. Aside from being generally
less annoying, it puts the least mechanical strain on the port,
and reduces the chance that you'll break the usb or serial port in
your laptop or other device. So if I have to use adapters, I try
to use a standard cable on each device, and stick any adapters in
cabling in the middle. Although, those mini adapters that are just
like two plugs back to back, a single one of those on an existing
plug isn't *too* bad.
I also don't like relying on things that are too custom that you
couldn't find from other sources or 15 years later.
But I myself spent some time hunting down the ususual "ideal"
serial cables to document on tandy.wiwk, so that you could have a
serial cable that had everything but the usb all in a single cable
with no adapters. Partly for the neatness and convenience, and
also to document something that a new user could buy and it "just
works" even if they have no idea how rs232 works. Those cables are
off-the shelf, not made custom by a single person, but they are
still non-standard cables that are really only good for this one job.
So, I'm guilty of violating that "standard interfaces" rule a
little myself, because those cables look like modem cables but
they're not. The really ideal minimal as possible arrangement, yet
that maintains all standard parts and interfaces all the way, is a
standard 9-25 modem cable, a standard usb adapter, and a 9m/9f
mini null-modem adapter in the middle. (Except I think no one
makes one of those mini-null modems with complete wiring, while
some of the cables I found are more complete.)
I have to admit, for the purposes of simplicity and removing all
the variables that people are mystified by... this does remove all
opportunities to get anything wrong, and makes the smallest and
simplest object to include in a laptop bag along with the M100. It
only does one thing, but if you're only doing this one thing, this
is the simplest way to do it.
I would put a label right on the plug that says For Model 100
though if I got one. Or at least one that says Port is DCE or
something.
It's bad to have objects lying around that look like one thing,
but are actually something else. If I didn't know the extremely
bizarre special purpose this object was wired for (let's say I'm a
young relative 10 years from now) and I came across it in a box of
junk, and tried to use it, it wouldn't work, I'd watse time
spinning my wheels trying things that *should* work, and then
throw the thing away. Or I'd stumble upon some combination of
adapters that worked, and then I'd think I leaned how serial ports
worked except what I learned wouldn't work anywhere else. (and
then if I somehow ever found out out that it wasn't broken, it was
just wired up in an insane way, I would curse whoever made it, and
whoever didn't put big explanatory labels all over it).
All that said, it's a cool find and I'll probably get one!
For an example of what I mean about not combining too much...
Say you don't buy into any of that nonsense I just bloviated so
much about, and you like this all-in-one cable because, duh, look
how slick that is! Right from the laptop to the m100 in one shot!
Now you want to use mComm on your phone instead Laddie on your laptop.
Now what? You need... an adapter. You need an "otg" usb adapter to
go from the phones usb-c or micro-usb to provide a usb-a port. Do
you really want to go back to that guy on ebay and have him make
another special cable with a micro-usb on the end instead of
usb-a? If not, why not? What was the point of getting this custom
cable in the first place?
And then next year when you get a new laptop that only has usb-c
ports. You need an adapter again, or you need yet another custom
cable whose only difference is a usb-c plug on the end...
Maybe I won't get one after all because both those last two
examples are already true for me. I have an android phone and an
android tablet with only a micro-usb. My next phone will probably
have usb-c. I already have old and new laptops with both usb-a and
usb-c, including one with only a single usb-c port.
--
bkw
On Thu, May 30, 2019, 4:38 PM Jim Anderson <[email protected]
<mailto:[email protected]>> wrote:
I ran across this eBay seller while looking for ham radio
programming cables for my various radios - the seller is a ham
and makes custom serial programming cables for hundreds of
varieties of radios. I did not think to look deeply at his
inventory when placing my order with him, but it occurred to
me today that he might also make different varieties of
regular DB-9 and DB-25 based USB serial cables, and what do
you know, he does!
One of the cables seems like it might be perfect for our hobby
- USB with an FTDI chip on one end, DB-25 male wired null with
full handshaking on the other end, and what seems to be a
fairly thin and flexible cable in between. The only downsides
are that it's a bit pricey ($37 US) and that the DB-25
connector shell he uses may be too bulky to properly fit a
Model 100 without cutting extra clearance on the connector
shell or the M100 housing (this is a known problem with some
cables and the 100, but doesn't affect the 102 or 200). Also
AFAIK he only sells on eBay, not through other retailers,
which may be a problem for some people.
https://www.ebay.com/itm/FTDI-USB-DB-25-Male-Serial-RS-232C-Null-Modem-Full-Han
dshake-Cable-NM-25M/151495441948
<https://www.ebay.com/itm/FTDI-USB-DB-25-Male-Serial-RS-232C-Null-Modem-Full-Handshake-Cable-NM-25M/151495441948>
http://tinyurl.com/y3wln2l3
I really wish I'd bought one while I was ordering cables from
him, even just to try it out (it would have been cheaper
because shipping to Canada is $$$ and I was getting three
cables sent already for the shipping price of one). If I have
a reason to buy anything else from this guy I'll definitely
get one of these and give it a whirl if nobody else has tried
and vetoed it by then.
jim
