In article , Dave Plowman (News) writes
High baud rate and no hardware handshaking, though 4m is not long for RS232. I'd suggest a good quality screened cable. The mic cable suggested by another poster should do the trick. Connect the screen to the plug body at the PC end.
In article , Clint Sharp writes
I've seen several installations using 9600 baud at a couple hundred metres, though at those lengths quality shielded cable is needed and the line drivers are more prone to failure due to induced noise. This was RS232, not one of the balanced variants.
That's a good idea even if it's also connected to pin 5?
In article , Dave Plowman (News) writes
Yes. pin 5 is the ground return for the serial data. Connecting the screen of the cable to the body of the D plug will ensure the cable is screened. You mention it being in a workshop, with the electrically noisy environment that implies.
Its better to keep signal ground and screen separate... so generally I use a core in a screened cable for signal ground, and save the screen for its intended purpose.
Might be worth noting that most PCs etc have ports with drivers based on the RS232C spec (lower voltage signalling) and not the full RS232 one. So results will vary with different kit depending on its spec.
I take it that one end of the cable has a flying braided lead to earth the screen then?
Dave
I don't actually have any 3 core screened.
As it happens mic cable works just fine. There was a fault in the wiring instructions for the serial to RG45 lead they supplied with the D socket loose - TX and RX reversed. Which to be fair was in the errata at the end of the instructions. ;-)
But many more do.
Gazillions of UARTS in microcontrollers will run way faster than that. I've done 1mbit/s in a PIC, between two devices on a PCB.
The crystal selection becomes critical at higher speeds if you want to use "standard" baud rates.
MBQ
We used to run the archive tape drives on System X at 880k over 150m of cable. We invented a new tape protocol just for the job as we hated the stupid ascii interface Tandberg had chucked together. That was in the '80s
On the 25 way connector pin 1 is reserved for screen, and pin 7 for signal ground. The screen is also in theory paralleled with the shell of the D connector.
Often happens - not helped by the swap over of TX and RX between connector sizes.
Indeed - but most are 8250(A) or 16550 style devices in PCs and similar. With the selected xtal these usually do about 50 - 115K bps.
Given a 68HC302 or similar with the embedded "communications element" you can do all manner of exotica with the serial lines!
Yup, there are not many choices if you want all the usual ones.
Thanks John, I don't have my spec handy at the moment and it has been a long time since I played with these conectors.
Dave
Don't think 8250 has been used in anything since the advent of 386's. Oldest I could find when sorting through Interactive UNIX's stock of really old test cards at work was a 16450 on an 8-bit ISA-bus card.
Southbridge chips contain embedded 16550A equivalents at least, and some turn out to be 16650 if you probe for them. Plug-in cards are
16650 through to 16950 (increasing buffer sizes, increasing numbers of features for high speed working with less driver intervention, but unfortunately increasingly less compatible between manufacturers). Very many cards nowadays use 8x the clock frequency a 16550 uses, with a divide by 8 which can be enabled/disabled to get to nearly a megabit, or to give the conventional 16550 speeds. (Sometimes the divide by 8 enabled/disabled applies to all ports on a multi-port card, which is a bummer as you can't set the baud rates independantly on each port to all of the commonly used values.)As I said earlier in the thread, modern UARTs are much more tollerant of mis-shaped signals and sample the data stream at 8 or 16 times the baud rate. This means they'll work with cable lengths well in excess of those in the original RS242 and V.24 specs.
You know what they say about RS232 - the 232 bit describes the number of valid ways you can connect three wires to a 25 pin plug ;-)
Yup they went out with the ark... I have a schematic from a PC-AT tech ref manual that does show one though (from the days where a 6MHz 80286 was the best you could get ;-)
The 16450 is not much better since it does not have a big enough fifo. So couple it with a OS with louse real time performance (like windows) and its useless.
I remember phoning round the multi serial card manufacturers a few years ago, trying to find one that would support 75 bps for use with older radio modems. It completely threw them off their sales pitch having someone not interested in how fast it could go, but how slow ;-)
Agreed. In fact even much of the older RS232 stuff would routinely go way beyond spec.
That's nowt - try looking for 45.45bps support :-)
Apparently the limitation is the number of clock divider stages in the UARTs. When they increase the clock speed to support higher data rates, they don't have enough dividers to get below 300bps.
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