Whence 556?

[Rich Alderson]

From: Shoppa, Tim
Sent: Sunday, June 03, 2018 10:35 AM

> The 729 CE manual quotes 555 BPI. I’m not sure when it became 556.

> http://bitsavers.trailing-edge.com/pdf/ibm/magtape/729/223-6845_729_CEman_1959.pdf

> The IBM 728 was 248 BPI. Before that it was the nice round number 200 BPI.

> I tried permutations of standard IPS and round number data rates and don’t
> see anything that yields exactly 556BPI.

Just to add to the mists and smoke and chaos:

For decades, I remembered the lowest density for magnetic tapes as 225 bits/in,
not 200.  I have never been able to find the number "225" in any manual since
starting the project which became the museum 15 years ago (sob!), but it still
hangs around in my mind and pings whenever I see "200BPI" mentioned.

Anyone else ever encounter that?

                                                                Rich


Rich Alderson
Vintage Computing Sr. Systems Engineer
Living Computers: Museum + Labs
2245 1st Avenue S
Seattle, WA 98134

mailto:RichA at LivingComputers.org

http://www.LivingComputers.org/