View Full Version here: : Computer lead lengths?

06-08-2017, 07:06 AM
After much deliberation, have decided my ob (complete) will roll off exposing the scope (12" newt on pillow block mount) on its pad. The control system will be a CNC lathe controller and stepper motors, in turn driven by EQMOD. The CNC system will be mounted on the pier and be locally controlled for centering and speed tuning with a gaming controller. However, I want to mount my main computer and screen on the wall of the ob. My question is (realising the shorter the better) what are the maximum effective lead lengths for a DB25 cable in this instance? Has anyone had experience similar to this scenario?

06-08-2017, 10:01 AM
Hi Rod,

You need to qualify your question more.

Firstly, DB-25 refers to a type of connector, not a type of cable.
There are a range of cables types of different construction that can be used
with a DB-25.

Secondly, what type of signals are you carrying on the cable?

When you mention a CNC controller I suspect you might be using one that
relies on an old parallel port interface. Would that be true?

In which case, you might be referring to the 25-way DB-25 connector that
PC's used for their Centronics/IEEE 1284 parallel ports to interface
printers but which was also to interface to some equipment such as
CNC controllers.

Some of the later interfaces would commonly use ribbon cable.

The rule of thumb for the length of a Centronics cable was 6'.

However, people commonly used shielded cables for interfacing to
printers that were as long as 25'.

So if you are using a parallel port interface to a CNC controller, I'd
recommend 25' max if you were pressed. 10' would be in a more comfortable
zone and 6' is guaranteed.

If using unshielded ribbon cable, watch out for the fast rise times of the
signals that travel down which can give rise to massive amounts of EMI.
You don't want this to couple by cross-talk into your imaging gear.

In previous lifetimes I managed a group of engineers designing
laser printer controllers for major US and Japanese electronics
companies and personally designed and tested many parallel ports.

So if you are using a parallel port, I hope the above is useful to your
decision making.

Best Regards

Gary Kopff
Member IEEE 39 years

06-08-2017, 12:08 PM
Thanks Gary for a concise answer! It is a parallel port. 6ft is going to be very tight, but now I can plan on using 10ft of the best shielded cable I can get hold of, and that will give me heaps of play. Full steam ahead again!:)

06-08-2017, 03:08 PM

A DB25 connector was traditionally used for RS232 serial comms. Depending on the type of cable and the baud rate this can work over surprisingly long lengths. 20m should not pose a problem at 2400 or 4800baud.

1200 bps (quite slow) over 30m is not uncommon.

Your application does not require a fast interface, so slower is better if you want to be sure it works.

If however this is a parallel (Centronics) interface like that used for printers vintage 1980, then the length is far more limited and 3m will be about it.

If you have any choice in the cable use a Belden one.

08-08-2017, 12:43 PM
Thanks Wavy.

3 metres is going to be it, and I'll look into the Belden one.