old skool serial comms demo (might be of use to someone)

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old skool serial comms demo (might be of use to someone)

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August 23, 2015 at 12:38 pm

#701705
6

General Lighting

@general-lighting

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broadband ADSL/VDSL on copper pairs or DOCSIS on coax-cable as well as the modern USB works along the same principles (but much faster).

The signalling here is 8 data bits, 1 stop bit and no parity with IA5 (ASCII) coding. The confusing bit is remembering that there is always a extra bit sent at the start (to synchronise the distant equipment as there is no clock sent along with the circuit) and everything is inverted and backwards…
August 23, 2015 at 12:41 pm

#975435
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tryptameanie

@tryptameanie

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What???????????
August 23, 2015 at 12:52 pm

#975433
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General Lighting

@general-lighting

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tbf you are probably way too young to remember this kind of stuff; it is like when I showed a picture of a 5,25″ floppy disk to some younger staff at work and I might as well have waved around dinosaur-bones and shouted “UG! UG! UG!” at the top of my voice…

to simply it slightly; every letter you type has a code number -with todays multicultural Unicode it is now two or more octets (bytes) but original computers used a single octet for each character. A blank space ‘ ‘ has code 32 decimal or 0x20 hex, and the ! has code 33 decimal or 0x21 hex.

RS232 uses a change from -12volts to +12 volts to send a stronger signal down what could be many tens of metres of cable (often to a computer terminal in another building or a noisy lineprinter). -12V is binary 0 and +12V is binary 1. I assume you know something about binary; from that it should be simple enough to work out how the letters are being transmitted…
August 23, 2015 at 1:02 pm

#975436
1

tryptameanie

@tryptameanie

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Thanks bud :). I’ve seen 5-25″ floppies before (think I was 6 and they were the height of technology and quite thin). I’ve seen alternating voltages etc. used as markers for binary in a few systems before, quite possibly from a few posts by a man called Clive Robinson on Bruce’s blog.
August 23, 2015 at 1:57 pm

#975434
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General Lighting

@general-lighting

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old style modems converted the binary data into various audio frequencies which are small alternating voltages (ADSL/VDSL do similar but at higher frequencies into the megahertz range). A very old skool way of using alternating voltage to signal on a telephone circuit is the ringing voltage – about 16,67 to 25 Hz at 90-100V AC (I had to use the smaller oscilloscope as it is impossible to turn the magneto handle and press the print screen button on the newer one at the same time)

the voltage is fairly strong so you don’t want to be on the wrong end of the terminals when a phone is ringing..

all analogue phones (even modern ones) work like this
August 23, 2015 at 2:06 pm

#975437
1

tryptameanie

@tryptameanie

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Hey GL I had actually been thinking of a system which you describe (with the voltages, or +/-, 0/1) but thought I’d dreamed it. Many people today see binary as a modern concept but if you look back through history you see binary used a lot, just not understood that the people are using a binary system. I suppose binary is a system you would use without knowing it than any other base system like termary, decimal, hexidecimal, etc.as it’s the yes or no option
August 23, 2015 at 4:20 pm

#975438
1

Mystic

@mysticgirls69

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Hmm, I’ll figure so,etching out of this ;).

Thanks much, I knew I could find it somewhere…
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