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The goal is to read the meter as fast as possible while the output is on. Using GPIB 16 to 18 readings per second is possible. Using RS-232 at 19.2kbaud the number of readings per second is 5 or 6. Here is a look at the code the meter sees:
read 70 ASCII characters of data
process data just read
} end loop
The computer is fast enough that the meter and RS-232 are the bottleneck. The loop is sequential so another INIT:IMM will not be sent until data from the previous FETC? has been read. The result is that communication is always one direction at a time. Since the length of the commands is short and only a single command is being sent at a time using XON/XOFF doesn't add any reliability and would just slow down communication. The computer input buffer is 512 bytes or more so the 70 character data set will never cause an overflow.
When 38.4k baud is used the number of reads per second is higher, though not double. Eventually the meter will fail to respond to one of the "FETC?" commands resulting in a read timeout. Investigation has shown that the meter will report an error when this occurs. Sending FETC? again will retrieve the data.
An external analyzer was connected to monitor the communication. The analyzer shows that what is happening is exactly what is expected except that at some point the meter fails to send the data back. The "FETC?" command sent just before that is the same as all the others except the meter fails to respond and the meter reports an error, -113 Undefined header.
As a test the "FETC?" was split into two parts to see if the meter was sensitive to incoming data timing. "FET" was sent followed by "C?" 1.1 seconds later. The meter did not report any errors.
If anyone else has resolved this your help would be appreciated.
(a read timeout occurs here after 1 second)
GPIB is an option we also use but is a bit overkill for a single meter. A 2410 with USB would be ideal.
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