Pertec Interface Cable Length

Jon Elson elson at
Mon Aug 12 10:13:10 CDT 2019

On 08/11/2019 08:00 PM, Douglas Taylor via cctech wrote:
> This is where the electrical engineer could help.  How do 
> you determine how long a cable the 74LS240 can drive?
Well, there are several considerations.  First, it takes 
some current to charge up the cable capacitance.  More 
current charges the capacitance faster, but also creates 
faster edges which cause more crosstalk.  Then, the data 
rate needs to be considered.  Mag tape data rates are not 
that high.  So, for 1600 BPI at 45 IPS, the data rate is 72 
K bytes/second, or about 14 us per byte.

Twisted-pair cable should have a little less capacitance, 
and it is supposed to reduce crosstalk, so should work better.

The most serious problem is when many data lines switch at 
the same time, it may contaminate the clock pulses and cause 
bytes to be dropped or added.

With the low data rates involved, proper delays to allow 
ringing to settle on the data lines and prevent short 
crosstalk pulses from affecting the clocks should make the 
system very tolerant of cable issues.  But, maybe some 
engineers didn't really optimize their logic for these problems.


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