[personal] UK Computer Museum on TV

[Roger Holmes]

> From: "Mike Hatch" <mike at brickfieldspark.org>

>
> TM4's were on the SDS9300, six of them, as I recall they were 10  
> track but
> something also says 9 track, after all it was 35 years ago !..

Thanks for that. Yes a long time ago.

> There were also TM4's fitted to an ICL (I think a 1901) machine up  
> in London
> but I only visited for service once (dirty contacts on the tape arm
> switches).

More likely a 1301. When the 1301s were being phased out they had to  
connect a pair of 7 track decks to the 1301s to transfer data to  
1900. If they'd had 1900s with TM4s they would surely have chosen  
that route instead.

I presume you mean the contacts with the dash-pot attached, not the  
switches at the end of arm's travel as these are fairly bullet proof.  
Now I've said that, they will of course fail next week!

I was chatting to a friend who was an ICT field engineer yesterday  
and he said 'we must clean the vacuum chambers sometime'. I asked him  
how often they should be cleaned and he said at the start of every  
morning shift. This made us both grin because they have not been  
cleaned since about 1977 and they are still not causing any problems,  
we can read tape last written at that time too, though the writing  
logic 'runs away' for some reason.

> There were also TM2 drives on a LEO3 #26 at Charles House in London
> maintained by ICL for running gas bills. I gather quite a number of  
> the Leo
> machines used TM drives.

They were popular on 1301s too, though no TM2 equipped 1301s have  
survived. They had four times the throughput of the TM4 decks as they  
used 8 data bits per frame plus 8 cyclic redundancy check bits. The  
tape moved as 150 inches per second too compared with just 75 on the  
TM4s giving 90k digits per second compared to 22.5k on the TM4. Still  
300 bits per inch on each track though, seems very low density by  
today's standard.

Roger Holmes.