Length of Cables for ESDI Hard Disk Drives

I have had a problem with some of my ESDI disk drives for a number of years which may have a very simple solution. When I use the drives in a BA123 box with an 11/73, the system functions correctly while I am using 7 quad boards or less. Whenever I add the extra boards, the READ operations on the Hitachi ESDI DK515-78 disk drives (Sigma RQD11-EC quad ESDI controller) take about twice as long. The WRITE operations are still just as fast. I have been looking at the problem again recently and while it has not yet been resolved, I now begin to suspect that the cables between the RQD11-EC controller and the drives may be too long. Just a guess is that the total length of the 34-pin cable is six feet (with the first portion to the first header being four feet and three more pieces of eight inches making a total of six feet to the last header) and the 20-pin cables (there are four with three currently in use) are about four feet each. In addition, the three ESDI drives currently in use run off their own PC power supply (in fact it is the actual power supply that came with the drives within a large raid set-up to be used with a VAX). I also salvaged the fans inside the box and each drive has its own fan. Normally, I only turn on one ESDI drive when the other ESDI drives are not being used for backup purposes. In addition, I have installed a heavy ground cable between the PC power supply and the BA123 box. In case anyone asks, the last drive on the 34-pin cable is correctly terminated and the other drives are not terminated. Since everything runs VERY well in the BA123 with the DK515-78 drives while I use seven or less quad boards, I have not tried to investigate much until recently since the extra boards are used so rarely it has not been a problem 99% of the time. And the fact that everything works so well has not led me to suspect the length of the 34-pin/20-pin cables. Does anyone have any suggested maximum cable lengths? Sincerely yours, Jerome Fine