7854 readout does not conform to specification? (7L5 problem)

Nenad Filipovic

Hi Everybody,

This one is for 7000 series readout experts, hope you're out there
somewhere. I have a properly working late model 7854 that I'd like to use
with my 7L5 (early serial with old digital board, successfully converted
for use in 7854, will post full instructions on this modification later as
officially these do not exist for the old 7L5). Everything works fine
except the damn level readout that I really, really need because I heavily
use the 7854 averaging and math functions. The problem can be split in two
1. Incorrect level readout in LIN mode - turning the attenuator should
produce the expected 200mV-100mV-50mV-20mV... sequence, but instead 7854
displays 2mV-1mV-5mV-2mV... and so on in circle (zeroes are omitted, units
and first digit are fine).
2. 7L5 readout driving circuitry can be slightly modified to fix the
mentioned problem, but although that fixes the display perfectly (and is
fully compatible with other mainframes using "classic" readout boards), it
breaks the ability of the 7854 storage to correctly obtain a scale factor
for acquired waveforms. So to summarize:
- If readout is broken in 2mV-1mV-5mV-2mV... way, 7854 acquire command (at
least) captures the same scale factor (2mV-1mV-5mV-2mV...)
- If readout is fixed to show 200mV-100mV-50mV-20mV..., 7854 acquire
command displays a warning and sets 1 as the default scale factor

I believe the problem is fully on the 7854 side, now for some gritty
technical details if you're still with me. 7L5 uses a non-standard (but
still technically perfectly correct) mode to display readout, using all 10
time slots (TS1-TS10). For example the word "200mV" in LIN mode would be
encoded in the following way:
ROW address: 3221115545
COL address: 0003110012
where digit position denotes a time slot (from left to right TS1-TS10). For
comparison word "-123dBV" in LOG mode would be:
ROW address: 2221115545
COL address: 0062346702
Quick reference to the address table:
Column address 0 in TS1 denotes a SKIP operation, it's obvious that the two
zeroes in 200mV are coded in TS5 and TS6 (non-standard format) as opposed
to standard format that uses TS1 to specify "add one zero" or "add two
zeroes" operational addresses. So if 7L5 does a SKIP in TS1, ROW address
should make no difference (as long as it's in 1-9 range). On mainframes
with classic readout boards (tested on 7633 and 7104) it indeed makes no
difference, but on 7854 it painfully breaks stuff:
- if ROW=2 in TS1, readout correctly displays 200mV-100mV-50mV-20mV... but
storage acquire fails to obtain the scale factor
- if ROW=3 in TS1 (7L5 factory default), readout breaks and displays
2mV-1mV-5mV-2mV... storage acquires the (broken) scale factor
- if ROW=4,5..9 in TS1, readout correctly displays 200mV-100mV-50mV-20mV...
but storage acquire fails to obtain the scale factor
7000/7854 readout specification makes no mention of such nonsense. To add
to the confusion, 7L5 service manual mentions Q2325 (page 2-19) that sets
ROW=3 in TS1 in LIN mode (although TS1 COL=0 which denotes SKIP), but with
no explanation. In LOG mode Q2325 is off and ROW=2.

I wonder if all 7L5s (including ones with new digital boards) suffer from
this problem in 7854. I already lost weeks trying to fix this and I'm quite
disappointed. Conversion to make 7854 capture 7L5 spectrums was piece of
cake, but this "tiny" readout bug is a nightmare if you make a lot of
average level measurements.

If anybody has any hints or extra documentation on 7854 readout, I'd
appreciate any help...

Best Regards,
Nenad Filipovic

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