Reads/write to the GPIB bus.
length is the size of the input buffer, default 50.
adr is the GPIB address of the device.
out_string is sendt before ONLINE waits for input from the bus.
Example 1: TTG-7 thermometer
* = gpib(4, , L0)
Example 2: Keithley 617
* = gpib( 6, ,"F1X")
Example 3: Lakeshore 330
* = gpib( 9, ,"cdat?"char(10)) get setpoint
* = gpib( 9, ,"sdat?"char(10)) get temperature
gpib( 9) = "s30"char(10) setpoint
Example 4: Lakeshore DRC-91A
* = gpib( 11, ,"ws"char(13)""char(10)) ERR27 K
* = gpib( 11, ,"wp"char(13)""char(10))
+290.00K
* = gpib( 11, gpib.out,"xd01"char(13)""char(10))
Wrote 463 bytes to gpib.out
Sections 3.1.13 contains some hints, if I/O problems occur.