Cable Lengths

I have a printer that is connected to the back of my Data General mini computer that uses serial db25 as a connections.  I use a db25 to rj45 converter so I can use the exsisting network cabling instead of running serial cables everywhere.  I need to know what the distance limitation of a serial cable is.  Or in this situation is the distance converted to the capabilities of the cat 5 cable?
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I have heard the maximum is 300ft.  I had also heard a max of about 60ft on a parallel cable.  I have run parallel successfully using two 6ft cables at each end and a 100ft straight through cable.
Interesting question. The maximum cable length would depend on transmission speed, cable properties and signal level. Cat5 cable should be good for pretty long distances at serial communication (RS232) speeds, at least the same length as allowed for LAN use. Problem might be signal level, but that's difficult to say much about. COM port output level is pretty high, like +- 12 V, but cable losses occur of course, and they depend somewhat on what is in the other end. I hooked two computers up in a serial link (running interlnk/intersvr at 115 kbps) that was 60 ft and it worked OK. This was a cheapo multiple TP telephone cable. I'll listen to othe comments with interest.
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Assuing that this interface is an RS-232 standard interface (which seems likely) you have a length/performance derating to be aware of.  In general, the specification calls for up to 75 ft. at 9600 bps.  Longer runs can be made but the speed will need to be reduced.  Shorter cables can run faster.

Remember, however, that this is WORST CASE.  In general your cable is better than the minimum required by the spec, your drivers have better output signals, your receivers are better, and your noise environment is not as bad as the spec permits.  You can also extend the distance by using better (i.e. more shielding and larger wire gauge that called for).

In general, however, CAT5 UTP cable is NOT THE BEST choice for RS-232 signals.  The difference is that RS-232 is SINGLE ENDED and 10BaseT network signals are DIFFERENTIAL.  The UTP cable is just that, UNSHIELDED TWISTED PAIR, which is excellent for DIFFERENTIAL signals but poor for RS-232 single-ended.

The best choice for RS-232 is SHIELDED multi-conductor 16 to 18 ga. stranded copper wire.
You can also use a device called a Short Haul Modem to extend your RS-232 distances as far as a mile over standard UTP cable.
You can reach 300m with RS232 ( it uses +12V/-12V) for log 0/1) and more than 1000m with simple converter RS232/current loop. (it uses 20mA current).
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