User manual

Prologix GPIB-USB Controller User Manual
9/15/2009 7
As mentioned earlier, an un-escaped CR or LF acts as the USB terminator. The
terminating CR or LF is removed and GPIB termination characters (specified by ++eos
command) are appended before transmitting data to instruments.
Any USB input that starts with the unescaped “++” character sequence is interpreted as
a controller command and not transmitted over GPIB.
When configured to do so – using the ++auto command or the ++read command –
characters received from instruments are transmitted to host. Unlike while sending data
to instruments, no character substitution is performed. The firmware buffers only one
character. The FTDI USB driver provides additional buffering of about 4KB. If a device
is talking and the buffers (firmware’s and driver’s) are full, the device continues to be in
TALK mode but is stopped from further sending data using GPIB handshaking.
The ++eot_char command may be used to detect GPIB EOI signal assertion.
7.1. Binary Data Transmission
Prologix GPIB-USB controller can send and receive binary data to and from GPIB
enabled instruments.
No special action is necessary to receive binary data from instruments. Any binary data
received from the instrument is transmitted over USB to PC unmodified, just as with
ASCII data. Since binary data from instruments is not usually terminated by CR or LF
characters (as is usually the case with ASCII data), you may want to use the
++eot_enable command to detect EOI indicating end of data. See ++eot_enable
command help for more details.
Special care must be taken when sending binary data to instruments. If any of the
following characters occur in the binary data -- CR (ASCII 13), LF (ASCII 10), ESC
(ASCII 27), ‘+’ (ASCII 43) – they must be escaped by preceding them with an ESC
character.
For example, to send the following (decimal) binary data:
00 01 02 13 03 10 04 27 05 43 06
it must be escaped as follows:
00 01 02 27 13 03 27 10 04 27 27 05 27 43 06
Further more, most instruments will get confused if GPIB termination characters, such
as CR or LF, are appended to binary data. Use ++eos 3 command to disable such
behavior. See ++eos command help for more details.