Document long-string options: `FTVL=STRING` waveform vs `lsi`/`lso` (40-char `MAX_STRING_SIZE`)

[kgofron]

Context

While integrating ioc-tasplot (PyDevice IOC for TAS scan plotting), we hit EPICS’s 40-character string limit when exposing file paths (~90 characters) through PVs.

Investigation showed this is expected EPICS behavior, but the right PyDevice record choice is not obvious from the README alone.

Observation: waveform with FTVL=STRING

PyDevice’s waveform support for FTVL=STRING caps each element at MAX_STRING_SIZE (40), matching EPICS base (epicsTypes.h):

// pydev_waveform.cpp
std::string cval = sval.substr(0, MAX_STRING_SIZE - 1);

That is correct for EPICS string waveforms (an array of fixed 40-char slots), but it is not suitable for a single long pathname.

Question: Could the README explicitly state that FTVL=STRING waveforms are limited to 40 characters per element, and are intended for arrays of short strings, not single long text fields?

What worked: lsi / lso (EPICS 3.15+)

Switching path PVs to lsi/lso with SIZV=255 and enabling pydev315.dbd worked well. PyDevice’s pydev_lsi.cpp / pydev_lso.cpp correctly use rec->sizv for storage.

Remaining friction was on the client side, not PyDevice:

• Plain caget TAS:Plot:FilePath_RBV → truncated at 40 chars (DBR_STRING)
• caget -S TAS:Plot:FilePath_RBV.$ → full path

See EPICS field modifiers ($) and EPICS tech-talk on Phoebus long strings.

Alternative: waveform with FTVL=CHAR

Historically, long text used a CHAR waveform as a byte blob (NORD = length). PyDevice appears to support this, but from Python it means returning byte lists, and Phoebus needs Format=String plus the .$ modifier. For text fields, lsi/lso seem simpler.

Question: Is there a recommended PyDevice pattern for “single long string” PVs today? Our conclusion: prefer lsi/lso over CHAR waveforms for path/error text — please confirm or correct.

Suggested documentation additions

1. Record choice table

   Goal	Record type	PyDevice notes
   Single long string (path, error)	lsi / lso	Requires pydev315.dbd; set SIZV; clients use .$
   Array of short strings	waveform FTVL=STRING	40 chars per element (MAX_STRING_SIZE)
   Legacy long text blob	waveform FTVL=CHAR	NORD = length; less ergonomic from Python
   Numeric plot data	waveform FTVL=DOUBLE	Works well

2. Note that dbpf / iocsh on lso strings may truncate at 40 chars; use template field(VAL,...) at load time, Python pydev() before iocInit, or CA clients with the .$ modifier.

3. Optional: mention that input lsi/lso readbacks may need field(SCAN, ...) (or I/O Intr) so CA clients see updates without an explicit process trigger.

Environment

• EPICS Base R7.0.7
• PyDevice (vendored in ioc-tasplot src/)
• Python 3.10
• Clients: caget/caput, Phoebus Display

Not asking for

A change to make FTVL=STRING hold arbitrary-length strings — that would duplicate lsi/lso and diverge from EPICS record semantics. Documentation and a clear “use lsi/lso for long text” recommendation would be enough.

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