F8 and a GPS -OT (HTML for drawing)
gene.heskett at verizon.net
Wed Jul 9 04:57:28 UTC 2008
On Wednesday 09 July 2008, Les wrote:
>Gene and others will correct whatever I get wrong...
> The original specification was for a serial interface to multiple
>devices. If you look at the entries below 0x20 on the ASCII table, you
>can gather such gems as DC1 and SEND and some others. These were meant
>to control multiple devices on a single serial port. This is part of
>the software specification. The hardware specification implemented two
>serial busses in a single connector. Although most hardware never
>implemented the second bus, instead treating the one as bidirectional,
>which reduced the effectiveness of RS232. Also most software folks
>outside of IBM and UNIVAC seldom implemented the full control spectrum.
> The line drivers were supposed to be cabable of 20ma loop
>current. This meant that the serial loop could pass through a chain of
>devices, each of which could receive and transmit data.
>You could select which device(s) were active by chosing device control
>codes which would act on specific hardware in the receiving device. Its
>primary use was for communications, driving data to modems and receiving
>data from modems.
> Generally most folks would ground the shield, which was sort of an
>error. What should happen is that the shield is the return path for the
>transmitted signal. from the DTE. Signal ground is the return path for
>the signal originating at the DCE. Thus properly implemented the two
>sides have different ground references. At the DCE (modem or printer
>for example) the signal ground carries the return path for the signals
>going to the DTE. the shield carries the signal return to the DCE.
>Thid implied that who ever was working on the line had to remember which
>line he was on. 200' of line in a high RF environment or high loss
>environment could build up quite a charge if it were not dealt with
>properly. Inside the equipment, the receiving block at the DCE would
>tie its input circuits between shield and a floating 5v signal. It was
>never really floating, as there would be a high value resistor between
>the shield and signal ground. The transmitting block of the DCE would
>be tied between DCE +5 and signal ground.
> At the DTE end its transmitting block would be tied to +5 and shield
>to ground, with signal ground connected by a high value resistor. Its
>receiving block would be between a floating 5V and signal ground.
> NOW all that is the theory. In practice shield ended up being
>connected to case grounds on both ends, and signal ground was used as
>the reference. This meant that you had some signal degradation on
>simultaneous transmit and receive, resulting in the buss being used
>alternately for send and receive. Also most manufacturers didn't want
>floating supplies, so that ended. Finally they balked at the additional
>$3.00 or so for the line driver bits, and used normal 7400 or DTL logic
>for the lines. the result was lower speeds, shorter distances, loss of
>noise immunity, and too many unused pins on the connector. In came the
>DB9, and a host of other hardware interfaces, out went the lines
>available for ringing (the anonymous test lines), and so went the story
>of slow, limited line length RS232.
> So basically all you need is the pinout in the bottom connector of
>the drawing (DB-9) or one of the other interfaces, short leads
>(generally <6') and you should be all set. There isn't much that can go
>wrong other than you may need a null modem connector to transpose the
>two serial wires. They are available all over the place, or you can
>simply open up the offending connector and swap them there.
> The original interface was a 25 pin device:
And that is a better description than I could ever have written. I wasn't aware
that some of the odd signals were actually a second data channel. I also do
not think any of that is covered in "Understanding Serial Communications" or
whatever it was that the Shack was selling as The Bible of rs-232 20 years ago,
and which I have a copy of lolling around here someplace.
Unforch, the hardware side is only half of the story, the handshaking to assure
good data is the other half, and that can get fairly complex too as it appears
there is more than 1 interpretation of the '7-wire' protocol now so widely used
with the db9 connector cuz even that leaves 1 empty pin. The RI is rarely
connected or used, although some modems may have exerted it back in the dim
reaches of time 2 decades ago.
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
A glass-enclosed isolation cell where newly promoted managers are
kept for observation.
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