(Classic Computers) HP 7970 1/2" 9-Track Reel-to-Reel Tape Drive

J. David Bryan jdbryan at acm.org
Fri Oct 6 13:10:37 CDT 2017

On Tuesday, October 3, 2017 at 20:58, Jay West via cctalk wrote:

> Wasn't there some deal where a M/E/F could drive it at 45ips but a 2100
> couldn't (next lower speed)?? 

You might be thinking of the limitation regarding the use of the 12992D 
Magnetic Tape Loader ROM with a 1000 M-Series and a 7970E (1600 bpi) 
operating at 45 ips.  This bootstrap loader used skip-on-flag 
(word-at-a-time) mode, and the data transfer loop could not keep up at tape 
speeds greater than 37.5 ips.  The faster E- and F-Series CPUs could run 
the bootstrap with the drive configured for the full 45 ips.

The 2100 and the 1000 M/E/F-Series machines had no such limitation if 
DMA/DCPC was used to access the tape drive.  DMA wasn't used in the boot 
loader ROM because the code wouldn't fit in the 64-word ROM size.

(That said, I've rewritten the 12992D bootstrap to tighten the timing 
loops, and it works fine on my 1000 M-Series and 7970E drive.  I think the 
original limitation was due to suboptimal coding.)

                                      -- Dave

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