GPIB Data Transfer Rate

[GPIB.user]

The maximum data rate in the original GPIB standard is about 1MByte/s.  I have a microcontroller connected to a NAT9914 but I don't know what the transfer data rate is.  For RS232 protocol, I could specify the baudrate, say 9600.  Is it possible to specify the data rate for the NAT9914; say for example 500kByte/s?  I read about the T1 delay, but not sure how to apply it if I need 500kByte/s data rate for example.

Any suggestion or comment would be greatly appreciated.  Thank you.

[Steven_T]

Hello GPIB.user,

The speed of the data transfers will depend on two things.

1.  The speed that the talker is placing data onto the bus.

2.  The speed that the slowest listener is accepting data.

GPIB was made to be extremely robust.  I'm sure you've read something about the handshaking lines (NRFD, DAV, NDAC) that make sure that all instruments get the data.  These do not toggle until all listeners have entered the correct states.  So if you have a very slow GPIB instrument on the bus that is actively listening (accepting data and toggling the listener handshaking lines NRFD and NDAC) it will slow down the entire communication process.

The T1 delay that you mentioned controls the minimum time that the talker will wait before placing data on the bus.  Even if all listeners are ready for more data, it will wait T1 before sending another byte.  However if one instrument is taking a long time to accept the data, the talker must wait until all instruments are ready for more data before sending another byte.

So, there is no way to specify the data rate except with the T1 delay, but it only controls part of the total picture.

I hope this helps,

Steven T.

[GPIB.user]

Hello Steven,

Thank you very much for the information.  I'm actually trying to figure out how fast I'm sending out or receive data through GPIB on the equipment that I'm going to design.  I'm supplying a 2MHz clock frequency to NAT9914.  I have yet to determine the T1 value, as I have no clue what value to set it to.  The maximum transfer rate that we could get from GPIB is 1 MByte/s.  How do I figure out the speed of the equipment that I'm designing?  What do I need to set in NAT9914 to get the speed of say 500 kByte/s for my equipment?

Thank you.  Any suggestion or comment would be greatly appreciated.

[Steven_T]

Hello GPIB.user,

To reach the transfer speed of 500 KB/s, there is no need to set any of bits in any registers.  This is the default state of the T1 delay.  However I need to warn you that you will not be reaching 500KB/s of pure data.

For each read that takes place, the bus must first get configured for specifying which nodes on the GPIB network are listeners and which one is a talker (requires that at least three configuration bytes get sent). 

See the T1 Delay section in the manual (chapter 5).

Thanks,

Steven T.

[GPIB.user]

Hello Steven,

I just pick 500kByte/s out of the air, as it's one-half of the maximum transfer rate.  I'm not sure if I understand, but does it mean if the input frequency is 2MHz and the clock period is 500ns, and using the default T1 delay (which is 2usec, right?), the maximum throughput of my equipment is 500kByte/s?  What T1 value do I need to set to get close to the maximum throughput of 1MByte/s.  If I want a slower transfer rate of say 100kByte/s, what T1 value do I need to set then?  Or is it even possible to determine the data transfer rate of my equipment.

I'm still stuck with the question "How fast is your equipment transfer data?"  Unlike RS232, I could answer that it transfer at 9600 baudrate.  For GPIB case, I don't know, maybe there really isn't a way to set the transfer rate?  hmm..  How am I going to sell the equipment without knowing how fast it transfer data..

Help.. Confused,
GPIB.user

[Steven_T]

Hello GPIB.user,

If you are looking to design equipment that has high GPIB performance, having the lowest possible T1 delay would help.  If you must post a data throughput specification, you should benchmark the communication to see what the actual data rate is.  If you look at other vendors that make GPIB instruments, you will see that they do not have any data rate specifications.  They mostly just say that they are IEEE 488 compliant.

The reason (as I said in my previous posts) is that the GPIB bus is shared by all instruments.  The communication will be dictated more by the instruments listening on the bus than the instruments talking.

I hope this helps,

Steven T.

[maulik710]

hello sir,

i want to know that how much baurd rate for 1bite and suppose i want to transfer 0 to 7bit that time how much baurd rate is requred ..

i m doing project on GPIB so i want to know that data transfer in detail how we can?? 

[Albert.Geven]

The transfer in GPIB is bytewise. Baudrate officially is datapackets per second. Defined for serial interfaces.

If you want bits per second multiply this by 8 for GPIB.

The NI gpib interfaces are capable of a few megabytes per second. But most instruments are much slower.

So with which instrument is more important than the theoretical speed of the interface.

[maulik710]

thank you sir...

 can u give me any king of example.. may be it will be easy for my project...

[crossrulz]

If you are using LabVIEW and VISA, look up the Serial Read and Write example.  Just remove the Configure Port and use a GPIB VISA session instead of a serial.