Talk:UNIBUS parity

18-bit mode

Would it be appropriate to mention 18-bit data on the UNIBUS here? I understand the KS10 and some of its devices could use the two parity bits for data. Maybe some of the 18-bit PDPs too? Larsbrinkhoff (talk) 12:40, 23 January 2019 (CET)

Yeah, good idea. This hack is mentioned in a number of places (e.g. MX15-B Memory Multiplexer, RH11 MASSBUS controller, etc). Once I update here, I'll link them in.

I wonder if this possibility was thought of up front, or if this hack was only developed later, when someone had a brainwave? Jnc (talk) 14:28, 23 January 2019 (CET)

OK, done - see what you think.

Circling back to this, I checked https://bitsavers.computerhistory.org/pdf/dec/magtape/tm03/TM03_UsersGuide.pdf and on page 2-16 it says it can do 18 and 36 bits. Page 1-8 talk about the bit fiddler, but doesn't specify how it works with >16 bits. https://gunkies.org/wiki/TM03_magtape_controller says it can have a "M8915 for use with a PDP-10". Larsbrinkhoff (talk) 16:28, 6 February 2025 (CET)

Also, I wonder about the RH11 in the KL10 front end - could it do 18-bit transfers over the front end's UNIBUS? ISTR that the RP's on that were double ported, and the RH10 could get too them too, so maybe no need for the 18-bit mode on the front end (and probably not high enough performance anyway). Jnc (talk) 21:43, 23 January 2019 (CET)

From looking at some 1080/1090 documentation, access to shared RP disks from the KL10 side was via an RH10/RH20, not via the -11; in any event, I didn't see any indication that DMA cycles on the UNIBUS can do 18-bit cycles to the KL10's memory. Jnc (talk) 03:26, 24 January 2019 (CET)