HP 8753E Programming And Command Reference Manual page 36

Marker-output commands and queried commands are output in ASCII
format only, meaning that each character and each digit is transmitted
as a separate byte, leaving the receiving computer to reconstruct the
numbers and strings. Numbers are transmitted as 24-character strings,
consisting of:
Sign
3 digits
Decimal point
15 digits
E
Sign
Exponent
When multiple numbers are sent, the numbers are separated by commas.
When number pairs are sent, the numbers are separated by a comma
and terminated with a line feed (LF).
HP-IB Programrm 'ng and Command Reference Guide
Digits to the left of the decimal point.
Standard decimal point.
Digits to the right of the decimal point.
Exponent notation.
Two digits for the exponent.