Add initial support for the V&A VA40B multimeter.
serial-dmm: Drop obsolete function prototypes.
serial-dmm: Increase timeout when scanning for DMMs.

We have to wait a bit longer than 1s for a valid DMM packet to arrive,
since for various DMMs some modes (Hz/% for example) the packets will
arrive a lot less often than in other modes. If the waiting period is
too short detection of the DMM will fail.
gmc_mh_1x_2x: Fixed sign and AC/DC for current measurements with Metrahit <= 16.
zeroplus: Support all 32 channels of 32 channel models

This will need some additional work when support is added for compression
modes since group D is disabled for RLE compression and C and D are
disabled for "double" compression.

Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
zeroplus: Add missing config_get for SR_CONF_CAPTURE_RATIO

Without this, latest pulseview gets an assert and dies.

Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
zeroplus: Ignore capture ratio if there is no trigger

If there is no trigger, don't try to capture anything before it. There
won't be any because we trigger immediately.

Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
zeroplus: Always set DONT_CARE_TRIGGERBAR to 1

Experimentation with the windows driver has found no situation where
this is set to anything other than 1. The zerominus software also never
sets this to anything other than one. Revert the code change made in
0ab0cb942f.

Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
zeroplus: Modify analyzer_read_start to just prep for bulk reads.

Let the capture loop manage which samples are thrown out rather
than throwing out two here.

Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
zeroplus: Add getters for memory configuration

This is needed at capture readback time to determine how many samples to read
in.

Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
zeroplus: Major rework of sample buffer processing

The sample buffer is a still a bit of a mystery, but this should help.
The variables in play:

triggerbar/ramsize_trigger - These two variables added together indicate
how many samples we want captured. ramsize_trigger - triggerbar
indicades how many samples must be captured. The ratio between the two
is determined by capture ratio.

memory_size - This indicates the number of samples in the circular
capture buffer. stop_address, now_address, and trigger_address are
pointers within the zeroplus that wrap based on this size.

now_address - The address that the zeroplus was about to write to when
it finished capturing, and now the address that will be read from when
reads are done from the capture buffer

stop_address - The address that the zeroplus last wrote to when it
completed capture.

trigger_address - The sample address for which the trigger occured.

status - This one is a bit tricky. Some testing has shown that if the
zeroplus has captured memory_size or less samples, the STATUS_READY bit
is set. For all captures generated with more samples than this,
STATUS_READY was cleared. However, boundary conditions are difficult to
test and values such as, memory_size + 1 have not been tested. We use
this to determine if the capture has wrapped through the sample buffer.

More testing is required, but this improves behavior in a number of
cases, specifically capturing sample amounts that are not a power of 2
of the sample buffer size. Before, random data was passed to libsigrok.

Signed-off-by: Russ Dill <Russ.Dill@gmail.com>
zeroplus: Add voltage threshold support

It doesn't currently mesh well with libsigrok, but at least the support is there.
rigol-ds: add a few more DS2000 models to the supported list
rigol-ds: fix waveform reception on DS2000 series

The ":WAV:DATA?" scpi command must be sent before calling
sr_scpi_read_begin().
scpi_usbtmc: fix reading of blocks bigger than the 2048 bytes buffer
serial: Improved docs.
README.devices: Cosmetics, consistency fixes, updates.
Add initial support for the UNI-T UT60G.
Add support for the Tenma 72-7750 (UNI-T UT60G rebadge).
serial-dmm: Support the Tenma 72-7745 via UT-D02 cable.

(it was already supported in uni-t-dmm via UT-D04 cable)