Pertec Interface Cable Length

Nigel Johnson nw.johnson at
Mon Aug 12 10:16:20 CDT 2019

Another consideration with the TC02 is the small buffer. I don't know 
what tape speed your drive runs at, but we lost a lot of sales to Dilog 
because of buffer overflow on some of the faster CDC dirves.  When we 
came out with the TC03, it had a larger buffer to handle this.


Nigel Johnson

On 12/08/2019 11:13, Jon Elson via cctech wrote:
> 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.
> Jon


Nigel Johnson

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