UNIBUS and QBUS termination - Revision history

Jnc: /* External links */ +Q-bus Expansion Concepts

2025-05-30T11:08:53Z

‎External links: +Q-bus Expansion Concepts

Jnc: /* External links */ uNote at Frain Research went offline; switch to iBiblio copy

2025-05-30T10:59:56Z

‎External links: uNote at Frain Research went offline; switch to iBiblio copy

Jnc: /* See also */ Link to DEC terminators

2023-11-28T12:32:21Z

‎See also: Link to DEC terminators

Jnc: /* External links */ +Backplane Expansion/Termination

2023-07-10T00:15:08Z

‎External links: +Backplane Expansion/Termination

Jnc: /* External links */ Another interesting one

2023-07-08T05:39:48Z

‎External links: Another interesting one

Jnc: Add section on QBUS termination, along with links to manuals

2023-07-04T15:14:32Z

Add section on QBUS termination, along with links to manuals

Jnc: /* SACK turnaround */ Noise can cause an unused grant

2022-04-20T14:44:42Z

‎SACK turnaround: Noise can cause an unused grant

Jnc: Write up 'SACK turnaround' issues in detail

2022-03-31T21:42:33Z

Write up 'SACK turnaround' issues in detail

Jnc: also the -11/04

2022-02-20T21:49:10Z

also the -11/04

Jnc: SACK turnaround is only _potentially_ debugging

2022-02-20T21:29:48Z

SACK turnaround is only _potentially_ debugging

@@ Line 51: / Line 51: @@
 * [http://www.bitsavers.org/pdf/dec/qbus/Digital_Microcomputer_Products_Handbook_1985.pdf Microcomputer Products Handbook] (EB-26078-41) - Chapter 52, "Introduction to Backplanes, Enclosures, and Expansion Cables" contains a good introduction to QBUS termination issues
 * [http://www.bitsavers.org/pdf/dec/qbus/Digital_Microcomputer_Interfaces_Handbook_1980.pdf microcomputer interfaces handbook] - configuration of multi-backplane systems is covered on pp. 68-73 (pp. 76-81 of the PDF)
-* [https://www.ibiblio.org/pub/academic/computer-science/history/pdp-11/hardware/micronotes/numerical/micronote35.txt Backplane Expansion/Termination] (uNote #035)
+* [https://www.ibiblio.org/pub/academic/computer-science/history/pdp-11/hardware/micronotes/numerical/micronote29.txt Q-bus Expansion Concepts] ([[MicroNote]] #029)
 * [http://www.bitsavers.org/pdf/dec/pdp11/1103/EK-LSI11-TM-002.pdf LSI-11, PDP-11/03 user's manual] (EK-LSI11-TM-002) - Appendix E, "H9270 Backplane Configurations" (pp. 157-158 of the PDF)
 [[Category: UNIBUS]]
 [[Category: QBUS]]

@@ Line 51: / Line 51: @@
 * [http://www.bitsavers.org/pdf/dec/qbus/Digital_Microcomputer_Products_Handbook_1985.pdf Microcomputer Products Handbook] (EB-26078-41) - Chapter 52, "Introduction to Backplanes, Enclosures, and Expansion Cables" contains a good introduction to QBUS termination issues
 * [http://www.bitsavers.org/pdf/dec/qbus/Digital_Microcomputer_Interfaces_Handbook_1980.pdf microcomputer interfaces handbook] - configuration of multi-backplane systems is covered on pp. 68-73 (pp. 76-81 of the PDF)
-* [http://web.frainresearch.org:8080/projects/pdp-11/micronotes/micronote35.txt Backplane Expansion/Termination] (uNote #035)
+* [https://www.ibiblio.org/pub/academic/computer-science/history/pdp-11/hardware/micronotes/numerical/micronote35.txt Backplane Expansion/Termination] (uNote #035)
 * [http://www.bitsavers.org/pdf/dec/pdp11/1103/EK-LSI11-TM-002.pdf LSI-11, PDP-11/03 user's manual] (EK-LSI11-TM-002) - Appendix E, "H9270 Backplane Configurations" (pp. 157-158 of the PDF)
 [[Category: UNIBUS]]
 [[Category: QBUS]]

← Older revision		Revision as of 12:32, 28 November 2023
Line 40:		Line 40:

	QBUS system which have multiple backplanes (up to three) formally require separate configuration rules from those which have only a single backplane. Notionally, termination must be provided at the start of the first backplane, and the end of the last. In practice, especially in single-backplane systems, it was usual to ignore termination requirements; such systems usually ran without noticeable problems.		QBUS system which have multiple backplanes (up to three) formally require separate configuration rules from those which have only a single backplane. Notionally, termination must be provided at the start of the first backplane, and the end of the last. In practice, especially in single-backplane systems, it was usual to ignore termination requirements; such systems usually ran without noticeable problems.
		+
		+	==See also==
		+
		+	* [[M930 UNIBUS terminator]]
		+	* [[M9302 UNIBUS terminator]]
		+	* [[TEV11 Terminator]]

	==External links==		==External links==

@@ Line 45: / Line 45: @@
 * [http://www.bitsavers.org/pdf/dec/qbus/Digital_Microcomputer_Products_Handbook_1985.pdf Microcomputer Products Handbook] (EB-26078-41) - Chapter 52, "Introduction to Backplanes, Enclosures, and Expansion Cables" contains a good introduction to QBUS termination issues
 * [http://www.bitsavers.org/pdf/dec/qbus/Digital_Microcomputer_Interfaces_Handbook_1980.pdf microcomputer interfaces handbook] - configuration of multi-backplane systems is covered on pp. 68-73 (pp. 76-81 of the PDF)
 * [http://www.bitsavers.org/pdf/dec/pdp11/1103/EK-LSI11-TM-002.pdf LSI-11, PDP-11/03 user's manual] (EK-LSI11-TM-002) - Appendix E, "H9270 Backplane Configurations" (pp. 157-158 of the PDF)
 [[Category: UNIBUS]]
 [[Category: QBUS]]

@@ Line 45: / Line 45: @@
 * [http://www.bitsavers.org/pdf/dec/qbus/Digital_Microcomputer_Products_Handbook_1985.pdf Microcomputer Products Handbook] (EB-26078-41) - Chapter 52, "Introduction to Backplanes, Enclosures, and Expansion Cables" contains a good introduction to QBUS termination issues
 * [http://www.bitsavers.org/pdf/dec/qbus/Digital_Microcomputer_Interfaces_Handbook_1980.pdf microcomputer interfaces handbook] - configuration of multi-backplane systems is covered on pp. 68-73 (pp. 76-81 of the PDF)
 [[Category: UNIBUS]]
 [[Category: QBUS]]

@@ Line 1: / Line 1: @@
 '''UNIBUS and QBUS termination''' are very similar, since in [[analog]] terms the two [[bus]]es are very similar (e.g. their use of the same transceiver [[integrated circuit|chips]], such as [[DS8641]]'s).
-Their terminators do a number of things:
+Their [[terminator]]s do a number of things:
-* they provide analog [[terminator|termination]] the otherwise un-terminated [[transmission line]]s of the bus (i.e. a [[resistor|resistance]] at the end of a transmission line that prevents reflections of [[signal]]s passing down it, 'bouncing' off its end);
+* they provide analog termination of the otherwise un-terminated [[transmission line]]s of the bus (i.e. a [[resistor|resistance]] at the end of a transmission line that prevents reflections of [[signal]]s passing down it, 'bouncing' off its end);
 * provide pullups (so those [[wired-OR]] [[broadcast]] transmission lines normally float at roughly 3 [[volt]]s, unless actively driven low by one of the boards plugged into the bus);
@@ Line 9: / Line 9: @@
 * in some cases, (such as the [[M9302 UNIBUS terminator‎]]), they do 'SACK turnaround' (an 'unused' [[bus grant]] from the [[Central Processing Unit|CPU]] is 'turned around' by the terminator, and returned to the CPU as the SACK signal); this is discussed at greater length below.
-The first is not ''absolutely'' necessary; this can be seen in small [[QBUS]] systems which can get away without it in small configurations. Whether it's needed on particular system is hard to predict, theoretically; the easiest thing is to just try it and see. Note: it ''may'' 'work' without it, but not be as ''reliable'' as with it.
+The first is not always ''absolutely'' necessary; this can be seen in small [[QBUS]] systems which can get away without it in small configurations. Whether it's needed on particular system is hard to predict, theoretically; the easiest thing is to just try it and see. Note: it ''may'' 'work' without it, but not be as ''reliable'' as with it.
 The second  is necessary, but can be provided anywhere on the bus; most [[UNIBUS]]/QBUS CPUs do so, with the notable exception of the [[KD11-D CPU|KD11-D]] of the [[PDP-11/04]], and the [[KD11-E CPU|KD11-E]]/[[KD11-EA CPU|-EA]] of the [[PDP-11/34]]-[[PDP-11/34A|34A]]. (In the [[PDP-11/40]], there's an oddball exception: the M981 UNIBUS jumper does it - but that's 'sort of' part of the CPU.)
@@ Line 15: / Line 15: @@
 (Note that the first and second are usually both done by a single set of components.)
-==SACK turnaround==
+==UNIBUS SACK turnaround==
 The full story of the third function, 'SACK turnaround', is somewhat complex. The need for it is, as mentioned, that an 'unused' bus grant, theoretically for a [[peripheral|device]]'s [[interrupt]], can, if not 'turned around' by the terminator, can cause the CPU to freeze up. This because if the CPU does not have an 'interrupt grant [[timeout]]' [[circuit]], then after such an 'un-needed' grant has been sent out, the CPU can ''potentially'' freeze up, waiting for the interrupt bus cycle to come in.
@@ Line 32: / Line 32: @@
 (SACK turnaround could potentially be used for a start-up 'safety check' for grant line continuity, where an un-requested - and thus 'un-grabbed' by any device - bus grant from the CPU on start-up is turned around by the terminator; this would verify that the grant line is continuous between the CPU and the terminator. Apparently no CPU does this, though.)
 [[Category: UNIBUS]]
 [[Category: QBUS]]

@@ Line 21: / Line 21: @@
 So SACK turnaround is therefore ''required'' by ''some'' UNIBUS CPUs, which do not have that circuit; this includes the [[KD11-D CPU]] of the [[PDP-11/04]] and the [[KD11-E CPU]] of the [[PDP-11/34]] (although the optional [[M8264 No-SACK Timeout Module]] can be used on both of these to add that functionality). However, most PDP-11 CPUs (including all QBUS CPUs, such as the [[KDF11 CPUs]]) have it, and will time out unused grants. (This takes somewhat longer than the SACK turnaround mechanism, which is why that is preferred.)
-How can a device not accept the grant of an interrupt that it requested? For example, if, having requested an interrupt, the device is reset ''just'' as the CPU is granting that interrupt, by the time the grant arrives at the device, it will have 'forgotten' that it was ever requesting an interrupt, and pass the grant on through. If no downstream (on the bus) device is requesting an interrupt, the grant will go unused - hanging a CPU which does not have a grant timeout circuit.
+How can a device not accept the grant of an interrupt that it requested? There are several possible causes for an unused grant. One possibility is that [[noise]] on a request line will look to the arbitrator (in the CPU) like a valid request. Another possible cause is that if, having requested an interrupt, the device is reset ''just'' as the CPU is granting that interrupt, by the time the grant arrives at the device, it will have 'forgotten' that it was ever requesting an interrupt, and pass the grant on through. If no downstream (on the bus) device is requesting an interrupt, the grant will go unused - hanging a CPU which does not have a grant timeout circuit.
 Machines which do not have a grant timeout circuit will function correctly ''most'' of the time without a terminator with SACK turnaround functionality; they are potentially vulnerable to the 'un-claimed grant' bug described above, though.