A cable longer than a 50 feet or so may not work properly at high speed. Much longer lengths sometimes work OK, especially if the speed is low and/or the cable is a special low-capacitance type and/or the electronics of the receiving end are extra sensitive. It is claimed that under ideal conditions at 9600 baud, 1000 feet works OK. One way to cover long distances is to install 2@ line drivers near each serial port so as to convert unbalanced to balanced (and conversely) and then use twisted pair cabling. But line drivers are expensive.
Another way to increase the distance is to try to cancel out much of the magnetic field created by the currents in the transmit and receive data wires: TxD and RxD. To do this, ground return lines, which have current which is roughly equal (but in the opposite direction) are placed next to the transmit and received wires. Twisted pair has the best cancellation. Some DEC terminals have two signal ground wires for this purpose. For example, one pair would be TxD and SG(TxD) where SG is signal ground. If you use ribbon cable, insure that the TxD and SG(TxD) wires are right next to each other. Similarly for the RxD.
If there is only one signal ground wire provided by both the PC and the terminal, it may be split into two wires in a twisted pair cable for this purpose. You might think that return currents will be equally split between the two signal ground wires. This would cancel out only about half of the magnetic field. But it's better cancellation than this because return current prefers the path of least impedance. The return path of a data signal (such as TxD) has the lowest impedance (due to lower inductance) if it flows back in the same twisted pair. Although I've haven't seen any experimental test results for this method, it should allow longer cable lengths.
Hardware Flow Control cables
If you expect to use hardware flow control (handshaking) you will likely need to make up your own cable (or order one made). Of course, if the connecters on the ends of a used cable remove, you might rewire it. See Installing DB Connectors. You will need to determine whether or not the terminal uses the DTR pin for this, and if not, what pin (or pins) it uses. The set-up menus may give you a clue on this since there may be an option for enabling "DTR handshaking" (or flow control) which of course implies that it uses the DTR pin. It may also use the DSR pin. See Hardware Flow Control for a detailed explanation of it. Older terminals may have no provision for hardware flow control.
Cable tips
The normal "straight thru" cable will not work unless you are using it as an extension cable in conjunction with either a null modem (crossover or file-transfer) cable or a null modem adapter. Make sure that the connectors on the cable ends will mate with the connectors on the hardware. One may use telephone cable which is at least 4-conductor (and possibly twisted pair). Shielded, special low-capacitance cable computer cable is best.
A kludge using twisted-pair cable
See also Overcoming Length Limitations. Although none of the EIA-232 signals are balanced for twisted pair one may attempt to use twisted-pair cable with it. Use one pair for transmit and another for receive. To do this connect signal ground to one wire in each of these 2 pair. Only part of the signal ground current flows in the desired wire but it may help. Due to the lower inductance of the twisted pair circuit (as compared to ground return current by some other path) more return (ground) current will confine itself to the desired twisted pair than one would expect from only resistance calculations. This is especially true at higher frequencies since inductive impedance increases with frequency. The rectangular wave of the serial port contains high frequency harmonics.
Cable grounding
Pin 1 (of a DB25) should be chassis ground (also earth ground) but on cheap serial ports it may not even be connected to anything. A 9-pin connector doesn't even have a chassis ground. The signal ground is pin 7 and is usually grounded to chassis ground. This means that part of the signal current will
