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Specifications for Long Cable for Connecting ST-7 to a Computer

I have tested this 100 foot cable with the ST-7 and in fact extended it to 160 feet by adding three commercial printer cables in series with it. It works with no attenuation of the speed of downloading from the ST-7. The cable should be made with a length of Belden cable # 8138. This is a cable consisting of 8 twisted pairs of wires within a single overall shield. The cable is notable because it has low capacitance. The shield should be connected at both ends to the shell or frames of the plugs as is usual practice.

The cable must be connected exactly as follows:
Pair 1 Pins 2 and 3 To pins 2 and 3
Pair 2 Pins 4 and 5 To pins 4 and 5
Pair 3 Pins 6 and 7 To pins 6 and 7
Pair 4 Pins 8 and 19 To pins 8 and 19
Pair 5 Pins 10 and 11 To pins 10 and 11
Pair 6 Pins 12 and 13 To pins 12 and 13
Pair 7 Pins 15 and 20 To pins 15 and 20
Pair 8 Pins 9 and 18 To pins 9 and 18

The connectors are standard DB-25, one male and one female. I do not know how long this cable can be made. I would think, from my experience, at least 160 to 200 feet. The cable is approximately $1.00 per foot on a 100 foot spool. It is available in a 500 foot length as well. I got mine from Newark Electronics.

As an added note in constructing this cable, I recommend using metal hoods on the DB connectors and very secure connection of the shield to a good ground on both ends. Grounding of the entire electronic system is very important to avoid electrical noise being introduced into the digital transfer. There are other cables which will work, of course. I have been told that Belden 9937 which has totally shielded pairs has been used up to 200 feet. It has many more conductor pairs than needed and so is a very heavy cable. A lighter weight "pigtail" must run from it to the camera to avoid strain on the camera itself. I have been told that cables of 100 feet or longer can be obtained as custom made units with the D connectors on each end. I had to pull mine through an underground conduit and so installed my own connectors.

It should also be possible to use commercial cables and extenders in series if they are of good quality. (IEEE 1284 standard is best) They should have good grounding especially where the plugs come together.

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