Subject:
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Re: Assembling the HandyBoard
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Fri, 27 Dec 1996 05:40:25 GMT
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Original-From:
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Fred G. Martin <fredm@media.mit.eduNOMORESPAM>
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Viewed:
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1620 times
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> I was getting similar behavior with my board. It wasn't clear to me
> that pins 2 and 3 in the serial cable from the PC to the interface
> board should be switched. Credit goes to Mike Ross for helping me figure
> this out.
>
> Once I switched these two pins, the downloader (dl) works as advirtised.
> I suppose this means that you should use a null modem cable between the
> PC and the interface board. (I haven't tried that; I still have wires
> on a breakout box doing the job.)
The whole issue of null (inverted) connections vs. straight-through is
incredibly confusing, because you must take into account the gender of
the connector (male pins or female pin sockets) in determining whether
it is a null connection.
Start out with the assumption that a computer has a male pin
connector. This is bad from an industrial design standpoint, because
a pin connector breaks easier than a socket connector, but
unfortunately it is the standard: computers are supposed to have the
male connector.
So then the attached device -- typically, a modem -- should have a
female connector. Back in the early days, these two pieces of
equipment were referred to as the Data Terminal Equipment (DTE) and
the Data Communications Equipment (DCE). The computer is the DTE and
the modem is the DCE. DTE is male, and DCE is female. This is the
original RS-232 specification.
Okay, now to connect a DCE (modem) to a DTE (computer), you just go
straight through. Nothing fancy. In the original RS-232 spec, both
pieces of equipment had mating 25-pin "DB" connectors. Transmit and
receive are done on pins 2 and 3 of these 25-pins, as follows:
DB-25 PIN DTE (computer -- male) DCE (modem -- female)
std. 2 transmit receive
3 receive transmit
7 common ground wire
So, the computer transmits to the modem on pin 2, and the modem
receives on pin 2. Reflexively, the modem transmits on 3, and the
computer receives on 3. I should mention, the Handy Board interface
is wired just like a modem (I'm trying my best to follow the
standards).
A null connection is required when you want to hook two computers
together. First off, since both computers have male connectors, you
need either a gender-changing adapter or a female-to-female cable.
Then, you need the "null connection," which swaps pins 2 and 3. You
want pin 2 of one computer connected to pin 3 of the other, and
vice-versa, so that they can actually communicate. Otherwise,
transmit is connected to transmit, and receive to receive, and nothing
good happens.
To further confuse matters, it should be pointed out that you can get
the null function either in the adapter or in the cable. Of course,
if you have both a null adapter and a null cable, you no longer have a
null connection!
So now in the early 1980's IBM introduced the PC-AT computer with its
new 9-pin serial port. They correctly realized that many of the 25
pins were no longer necessary and why not use a smaller connector.
They continued the standard of the male-DTE-computer and
female-DCE-modem-or-other-attached-device, but, for an unknown reason,
decided to swap the functions of pins 2 and 3. (Also, ground moved to
pin 5, but that does not introduce the level of confusion.) So, for
the 9-pin IBM-AT "standard," we have:
DB-9 PIN DTE (computer -- male) DCE (modem -- female)
std. 2 receive transmit
3 transmit receive
5 common ground wire
There aren't many 9-pin DCE (modem) devices in the world, but there
are now *lots* of 9-pin male serial ports on computers, and lots of
25-pin female modems. So, to connect a 9-pin male serial port to a
the standard modem, you must connect pin 2 to 3, pin 3 to pin 2, and
pin 5 of the DB-9 to pin 7 of the DB-25 (the ground connection).
Now, should we consider this a null connection? I think not, because
it's the "normal" way of connecting a male DB-9 to a female DB-25.
Null connections are what happen when you must connect two computers
together or two modems together. And any normal "Hayes modem cable
for the IBM-AT" has the correct wiring.
Don't even get me started on the Macintosh. There two kinds of
minidin-8 to DB-25 male cables: one that connects to modems and one
that connects to printers. The early Mac people made a sensible
choice by decreeing that the computer should have the less breakable
female connectors, but then they decided that printers and modems
should have opposite wiring (even though both would have DB-25 female
jacks). Hello?
The simple thing to remember is that if you plug a modem cable into
the Mac, it gives you a 25-pin male end that makes the Mac look like
the original DTE standard for a computer. So it's easy to connect the
Handy Board to the Mac, you just get a modem cable and it works.
To summarize:
25-pin male to Handy Board: straight-through 25-male to 25-female
cable.
9-pin male to Handy Board: normal IBM-AT standard 9-female to 25-male
modem cable. pins 2 and 3 are swapped; pin 5 of DB-9 goes to pin 7 of
the DB-25. Fred doesn't think this should be called a null cable,
even though it does swap, because it's the normal way of connecting a
normal modem to a normal modern computer.
Macintosh minidin-8 to Handy Board: Modem cable for the Mac.
Happy holidays,
Fred
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Message is in Reply To:
 | | Re: Assembling the HandyBoard
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| (...) I was getting similar behavior with my board. It wasn't clear to me that pins 2 and 3 in the serial cable from the PC to the interface board should be switched. Credit goes to Mike Ross for helping me figure this out. Once I switched these two (...) (28 years ago, 27-Dec-96, to lugnet.robotics.handyboard)
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