Category Archives: HP 16500A/B

HP 16534A triggering problems

One of my HP 16500B logic analysis systems has a 16534A digital oscilloscope module installed. The module has a great specification: two channels, 2Gsamples/sec, 500MHz analogue bandwidth, and 32k record length. I would find it a really useful debugging tool, if it worked properly. Sadly it doesn’t.

Though it displays a trace successfully, and the controls work, it won’t trigger. The only way I get a trace is by auto-trigger. In this case the trace isn’t synchronised, so it’s impossible to catch a particular event at a particular time. Here it is displaying a square wave (well, it’s nearly square – I hadn’t adjusted the probe compensation so it’s a bit curvy on the top and bottom). Adjusting the trigger level just doesn’t make any difference.

The service manual for the 16534A isn’t very helpful. It describes roughly what the self-tests do, and how to run the self-calibration procedure, but has no detailed circuit information. The self-test reports a ‘D/A’ failure:

and the self-calibration procedure fails on the Trigger and Hysteresis sections:

The service guide describes the D/A test like this:

Test DAC This test verifies the correct operation of the D/A convertor on the board.
Both the offset and trigger level D/A convertors for each channel are set to a reference
level and then changed. The logic trigger IC is programmed to detect the changes. The
detection of a correct trigger indicates that the D/A convertor is operating normally.

It seems clear that the offset D/A converter is working, since the offset control works and moves the trace up and down as it should. However, the trigger level control doesn’t seem to be doing what it should. I had a look at the board, looking for components likely to be the DAC or trigger IC. Unfortunately, the semiconductors on there seem to be mostly either standard ECL logic, trivial things like op-amps, or magic custom HP chips with dozens of pins. One exception was this:

It’s an AD96687 from Analog Devices, which is a dual high-speed comparator with ECL outputs. If I was designing a high-speed trigger circuit using 1994-vintage components, I might use a device like this. I’ve had a look at all its pins with the power switched on and a signal applied and tests running, and none of them seem to change at all. One noninverting input is stuck at +5V, and the other inputs are near ground. One set of outputs isn’t even showing valid ECL logic levels, but it may be unused and thus have no load resistors on it.

Maybe it’s a red herring and nothing to do with the triggering at all, but I’m suspicious that I never see anything change. Without more circuit information, it’s hard to get much further, sadly.

HP 16500A memory upgrade part 7

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In part 6, I established that something was stopping the floppy disc drives in my 16500A working when I changed the ROMs to software version 00.02. Thanks to the help of some on the hp-agilent-equipment Yahoo group, especially Glen Slick, I have determined that there’s an important difference between early CPU boards running 00.00 and later ones running 00.02: the floppy disc controller chip has changed!

The old board has an FDC9793 and the new one a uPD765 (or at least its Zilog equivalent). There are pictures of all the boards here. Now, I started to wonder, could I somehow fit the uPD765 to my board? It seems crazy and would only make sense if it wasn’t possible to exchange the whole board. So I started investigate. As luck would have it I have a functional 16500B with a logic analyzer plugin, so I could have a look and see what was going on around the disc controller on my CPU board. I wired up the relevant pins of the FDC9793.

Examining the logic analyzer trace, it seems that the CPU is trying to read something from the disc controller, which is at least a start.

It’s reading from what would be the status register repeatedly, as if waiting for some condition to be true. It gets a value of 0x04 every time, which is presumably not what it wants. It does this at the right stage of the self-test. I thought this was promising, since it is at least addressing the chip. Might it be possible to wire one in place of the other? Their pinouts are different. Comparing them, using their datasheets:

FDC9793				uPD765
1				RESET	I	active high
2	nWE	I		nRD	I
3	nCS	I		nWR	I
4	nRE	I		nCS	I
5	A0	I		D/nS	I	data/nStatus register select
6	A1	I		D0	I/O
7	D0	I/O		D1	I/O
8	D1	I/O		D2	I/O
9	D2	I/O		D3	I/O
10	D3	I/O		D4	I/O
11	D4	I/O		D5	I/O
12	D5	I/O		D6	I/O
13	D6	I/O		D7	I/O
14	D7	I/O		DRQ	O	DMA request
15	STEP	O	Step output	nDACK	I	DMA ack
16	DIRC	O	Direction (high=in, low=out)	TC	I	end of DMA transfer
17	EARLY	O	write precompensation	IDX	I	index pulse
18	LATE	O	write precompensation	INT	O	interrupt output
19	nMR	I	master reset	CLK	I	8MHz
20	GND			GND
21	+5V			WCK	I	write clock
22	nTEST	I	tie to +5V	RDW	I	read data window
23	HLT	I	input, head engaged	RDD	I	read data from data sep
24	CLK	I	1MHz in	VCO/SYNC	O	enable VCO
25	RG	O	read gate to data sep	WE	O	write enable
26	RCLK	I	read clock from data sep	MFM	O	MFM/FM
27	nRAW READ	I	raw data from drive	HD	O	head select
28	HLD	O	head load	US1	O	unit select
29	TG43	O	track greater than 43	US0	O	unit select
30	WG	O	write gate	WDA	O	write data
31	WD	O	write data	PS1	O	precompensation shift (early/late/normal)
32	READY	I	ready	PS0	O	precompensation shift (early/late/normal)
33	nWF	I/O	write fault/vfo en to data sep	FLT/TR0	I	track 0/fault
34	nTR00	I	track 0	WP/TS	I	write protect/two side
35	nIP	I	index pulse	RDY	I	drive ready
36	nWPRT	I	write protect	HDL	O	head load
37	nDDEN	I	double density	FR/STP	O	fault reset/step
38	DRQ		data req, open-collector	LCT/DIR	O	low current/direction
39	INTRQ		interrupt req, open-collector	nRW/SEEK	O	indicates seek/read/write mode
40				+5V

There are some important differences here. Firstly, the uPD765 expects an 8MHz clock where the FDC9793 used a 1MHz clock. That’s not a good start. The uPD765 includes drive and side select logic, but the FDC9793 doesn’t. The two also differ a lot in how they handle things like write precompensation signals, and the uPD765 effectively multiplexes various signals too.

At this point I think the swap is just too difficult. It becomes cheaper and easier to swap the whole CPU board, if I can find one, or simply upgrade the whole machine to be a 16500B.

HP 16500A CPU board versions

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As part of my quest to upgrade my HP 16500A logic analyser’s memory, I’ve found out more than I ever expected about the various versions of the 16500A CPU board. I’ve been fortunate to have the help of the good people of the [hp_agilent_equipment] Yahoo! group. My most recent stumbling block has been that installing ROM version 00.02 in my (early) 16500A stops the floppy disc drives working.

Glen Slick, from the above group, was kind enough to send me high-resolution scans of the two 16500A CPU boards he has. That makes three known versions, and here I’ll post pictures of them and attempt a description of the differences.

16500-66503 with 1MB RAM

This is the board from my machine. It has 1MB RAM in 8 TMS44C256-12N DIL chips, and the floppy disc controller is a Standard Microsystems FDC9793. The ROMs contain software version 00.00. For completists, I have put images of the V00.00 16500A ROMs here.

16500-66507 with 2.5MB RAM

This is one of Glen’s boards. It has 2.5MB RAM fitted as 20 off TC514256AZ-10 1Mbit ZIP chips, and the floppy disc controller is a Zilog Z0765A (which seems to be compatible with the very common uPD765 as found in the original IBM PC). The ROMs contain software version 00.02. It also has three extra programmable devices with labels on them. The details of their encapsulation mouldings look very similar to PALs elsewhere on the board, but why don’t all the PALs have labels on them? The labelled ones are:

16500-80006, U86, by the floppy drive connector at the bottom
16500-80007, U104, top right
16500-80008, no designator visible, next to the floppy controller IC

16500-66510 with 4MB RAM

This is another of Glen’s boards. The part number (66510) seems to be on a sticker. The only noticeable difference, other than the RAM chip complement, from the 66507 board is that the chip top right is labelled 16500-80013. It has 4MB of RAM as 8 off TC514400AZ-80 4Mbit ZIP chips. The ROMs contain software version 00.02.

HP 16500A memory upgrade part 6

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In part 5, upgrading the software on my 16500A’s CPU board to version 00.02 resulted in a self-test failure, which wasn’t fixed by downgrading back to 00.00. I’ve just had a look to see why that was happening.

The front panel is connected to the backplane by a 10-way connector. I traced out the pinout of the connector:

+5V power
+12V power
+5V power
+12V power
Ground
Data
Ground
Data
Ground
On/off switch, ground to switch system off

There was no activity at all on pins 6 and 8, the data pins, even during self-test. I traced them through to the CPU board via pins 59 and 60 of the CPU board’s backplane connector, and thence to U90 pins 18 and 19. This is an HP custom chip. There seemed to be activity on many of its pins, but nothing on those two. Poking around, I found that two of its pins are connected to a nearby ceramic resonator which clearly wasn’t resonating. Aha! Got it. One of the pins of the resonator had broken. I soldered it back together and now the self-tests pass!

But there’s a fly in the ointment. V00.02 software doesn’t seem to be able to access the floppy disc drives, simply reporting ‘No Disc’ for both of them even when they’ve got discs in.

Looking at the floppy disc connector with a scope, the ‘drive select’ pins are definitely active. It seems that the pinout of Sony 3.5″ floppy disc drives changed at some point. The details are here:

http://www.retrotechnology.com/herbs_stuff/drive.html#35new

Pin#  Signal (Old Sony)  Signal (New Sony)  Pin#  Signal (Old Sony)        Signal (New Sony)
1     Disc Change Reset   NC                2     Disc Change Indicator    NC
3     +5V                 Key               4     Density Bit              NC
5     +5V                 Ground            6     Drive Select             NC
7     +5V                 Ground            8     Index Pulse              Index
9     +5V                 Ground            10    Drive Select 0           NC
11    +5V                 Ground            12    Drive Select 1           Drive Select 1
13    Ground              Ground            14    Drive Select 2           NC
15    Ground              Ground            16    Motor On                 Motor On
17    Ground              Ground            18    Direction Select         Direction
19    Ground              Ground            20    Step                     Step
21    Ground              Ground            22    Write Data               Write Data
23    Ground              Ground            24    Write Gate               Write Gate
25    Ground              Ground            26    Track 0 Indicator        Track 00
27    Ground              Ground            28    Write Protect Indicator  Write Protect
29    +12V                Ground            30    Read Data                Read Data
31    +12V                Ground            32    Head Select              Head 1 Select
33    +12V                Ground            34    Ready                    Disc Change

It seems that the ‘disc change indicator’ has moved at some point in the history of Sony disc drives. Maybe the new software is expecting to see the ‘disc change’ indicator before it tries to switch the drive motors on? Grounding pin 34 doesn’t seem to make any difference. The disc controller is an FDC9793. Looking up the data sheet on it, it doesn’t have any means of indicating whether a disc is in or not. Without a schematic, it’s going to be very hard to figure out how the software determines it.

HP 16500A memory upgrade part 5

HP 16500A memory upgrade part 4

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Having worked out the wiring for a 4MB SIMM to the 16500A CPU board in part 3, I got out the mod wire and soldered it all together.

It all went together quite easily. The pins, even on the SIMM, are so much further apart than modern surface-mount components that I’m used to dealing with. Notice the red wire running along the row of chips on the SIMM: that’s nOE, where I had to lift pin 16 of each chip and wire them together because the original SIMM tracks grounded them. I also cut the track joining all the pin 3’s (nWE) of the chips together, half way along the SIMM, so that the upper four chips had a separate nWE from the lower four, again because that’s the way the CPU board expected them.

Having checked and double-checked the wiring, I plugged the board into the mainframe and switched on. Much to my amazement, I heard the sound of the floppy drives seeking almost immediately. It was running! Once the monitor had warmed up, I could see that the mainframe was booting absolutely normally. It passed all its self-tests. But, did it recognise the extra memory? With some trepidation I stabbed the screen to select test mode and looked at the bottom line of the display:

Oh dear. Just 1MB then. Serves me right for trying to make a silk purse out of a sow’s ear. I suspect ROM version 00.00 (what kind of a version number is that?) doesn’t support more than this, since later 16500As had a different ROM version and more memory. Just for kicks, I popped in the test disk and tried the proper memory test:

At least the new RAM works as well as the old stuff did, even if it isn’t any bigger. I haven’t broken it, but I haven’t improved it either. I also tried loading the 16550A module software, just in case it was somehow able to find the extra RAM even if the rest of the system didn’t know about it, but it still reported that there wasn’t enough space.

I think my next quest will be to find a copy of the later ROM and program a couple of EPROMs with it. A quick Google search for ‘16500A ROM’ reveals this site:

http://www.jammarcade.net/hp-16500a-logic-analyser/

with an image of the later ROMs. When I get a moment, I’ll program up a pair of 27C256As and see what happens…

HP 16500A memory upgrade part 3

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I’ve worked out the wiring of the SIMM to the CPU board. Unfortunately, the OE signal on the SIMM is hard-wired to ground, which is annoying. I’ll have to lift the legs (pin 16) of each of the RAM chips and hand-wire them together.

This is completely untested…

	Signal	16500A chip	pin	SIMM signal	SIMM pins
.	GND	all	20	VSS	1, 39, 72
.	Vss	all	10	VDD	10, 30, 59
.	D0	U43, U52	19	DQ8, DQ24	26, 27
.	D1	U43, U52	18	DQ7, DQ23	24, 25
.	D2	U43, U52	2	DQ6, DQ22	22, 23
.	D3	U43, U52	1	DQ5, DQ21	20, 21
.	D4	U23, U34	2	DQ4, DQ20	8, 9
.	D5	U23, U34	1	DQ3, DQ19	6, 7
.	D6	U23, U34	19	DQ2, DQ18	4, 5
.	D7	U23, U34	18	DQ1, DQ17	2, 3
.	D8	U44, U53	2	DQ16, DQ32	65, 64
.	D9	U44, U53	1	DQ15, DQ31	63, 62
.	D10	U44, U53	18	DQ14, DQ30	61, 60
.	D11	U44, U53	19	DQ13, DQ29	57, 58
.	D12	U24, U35	19	DQ12, DQ28	55, 56
.	D13	U24, U35	18	DQ11, DQ27	53, 54
.	D14	U24, U35	1	DQ10, DQ26	51, 52
.	D15	U24, U35	2	DQ9, DQ25	49, 50
.	W0	U23, U34, U43, U52	3	nWE	47 (top half)
.	W1	U24, U35, U44, U53	3	nWE	47 (bottom half)
.	RAS	all	4	RAS0, RAS2	44, 34
.	CAS0	U23, U24, U52, U53	17	CAS3, CAS2	42, 41
.	CAS1	U34, U35, U43, U44	17	CAS1, CAS0	43, 40
.	OE	all	16		wire separately
.	A0	all	6	A0	12
.	A1	all	7	A1	13
.	A2	all	8	A2	14
.	A3	all	9	A3	15
.	A4	all	11	A4	16
.	A5	all	12	A5	17
.	A6	all	13	A6	18
.	A7	all	14	A7	28
.	A8	all	15	A8	31
.	A9	all	5	A9	32

HP 16500A memory upgrade part 2

PIN	SYMBOL	PIN	SYMBOL	PIN	SYMBOL	PIN	SYMBOL
1	Vss	19	A10	37	NC	55	DQ12
2	DQ1	20	DQ5	38	NC	56	DQ28
3	DQ17	21	DQ21	39	Vss	57	DQ13
4	DQ2	22	DQ6	40	CAS0#	58	DQ29
5	DQ18	23	DQ22	41	CAS2#	59	Vdd
6	DQ3	24	DQ7	42	CAS3#	60	DQ30
7	DQ19	25	DQ23	43	CAS1#	61	DQ14
8	DQ4	26	DQ8	44	RAS0#	62	DQ31
9	DQ20	27	DQ24	45	NC/RAS1#*	63	DQ15
10	VDD	28	A7	46	NC	64	DQ32
11	NC	29	NC (A11)	47	WE#	65	DQ16
12	A0	30	VDD	48	NC	66	NC
13	A1	31	A8	49	DQ9	67	PRD1
14	A2	32	A9	50	DQ25	68	PRD2
15	A3	33	NC/RAS3#*	51	DQ10	69	PRD3
16	A4	34	RAS2#	52	DQ26	70	PRD4
17	A5	35	NC	53	DQ11	71	NC
18	A6	36	NC	54	DQ27	72	Vss

HP 16500A memory upgrade part 1

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My HP 16500A has only 1MB of RAM, which isn’t enough to run the system software for the 16550A logic analyzer module I’d like to use in it. Apparently there was a later version of the 16500A which had 2.5MB of RAM, but I haven’t got one of those. Just my luck. Trying to load the 16550A system software results in an error message:

This got me thinking. Might I be able to upgrade the memory? The RAM on the 16500A processor board is eight 1Mbit DRAM chips, soldered in:

I looked up the data sheet for the chips, TMS44C256. It indicates that pin 5 isn’t used. However, on larger (4Mbit) chips, pin 5 is another address line. Intriguingly, pin 5 is wired up on this processor board, implying that the hardware is ready for larger RAM chips. I have no idea whether the software is ready, but there’s only one way to find out. It’s got to be worth a try, hasn’t it?

Unfortunately I couldn’t find any 4Mbit DRAM chips in the DIL package to fit this board, and making or buying adapters for surface mount chips seems like a pain. However, there is another approach: old PCs are full of SIMM memory modules which have electrically suitable chips on them, and they’re already wired conveniently together. If I removed the 1Mbit DRAM chips from the processor board and string wires from the resulting holes to a SIMM, I might have a 4MB 16500A. Or a doorstop.

To change DRAM chips requires knowing how the address and data buses are wired, and how the RAS and CAS strobe lines are connected to the various chips. RAS and CAS are effectively chip enables, so they can be considered to be part of the address for wiring purposes. A bit of reverse engineering with a continuity tester resulted in this sketch of how the RAM is wired up:

If I do a similar job on a suitable 72-pin 4MB SIMM, it should be possible to match up the wires and see what happens. There’s a handy vintage SIMM data sheet I found here:

http://www.pjrc.com/mp3/simm/datasheet.html

which should be a good start. What can possibly go wrong?

HP 16500A and 16500B logic analysers

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Category Archives: HP 16500A/B

HP 16534A triggering problems

HP 16500A memory upgrade part 7

HP 16500A CPU board versions

16500-66503 with 1MB RAM

16500-66507 with 2.5MB RAM

16500-66510 with 4MB RAM

HP 16500A memory upgrade part 6

HP 16500A memory upgrade part 5

HP 16500A memory upgrade part 4

HP 16500A memory upgrade part 3

HP 16500A memory upgrade part 2

HP 16500A memory upgrade part 1

HP 16500A and 16500B logic analysers