PDP-12 Restoration at the RICM

Rich Alderson RichA at LivingComputerMuseum.org
Tue Oct 13 13:28:40 CDT 2015

From: Paul Koning
Sent: Tuesday, October 13, 2015 7:55 AM

>> On Oct 12, 2015, at 9:16 PM, Rich Alderson <RichA at LivingComputerMuseum.org>>
>> wrote:

>> ...
>> The M tracks are longitudinally encoded (6-bit values chosen such that
>> they read the same as NRZ backwards and forwards for DECtape, 4-bit
>> values for LINCtape) to predefine blocks (cf. disk sectors) for data.

> More precisely: it's Manchester encoding, not NRZ.  The result is that
> mark track codes are complemented and reversed end for end if you read
> them in the opposite order.

OK, what I had readily to hand was the TC02 DECtape Control maintenance
manual (from the PDP-9 series, DEC-09-I3CD-D), where page 2-3 states:

    Data is recorded by the Manchester method in which a prerecorded
    timing track synchronizes read/write operations.  When writing on
    the tape, the write amplifiers supply the maximum current in either
    one direction or the other (non-return to zero, NRZ).

So yes, you're correct to observe that NRZ is not as important as Manchester
in a description of *reading* the data.  Thank you for the correction.


Rich Alderson
Vintage Computing Sr. Systems Engineer
Living Computer Museum
2245 1st Avenue S
Seattle, WA 98134

mailto:RichA at LivingComputerMuseum.org


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