Two 7890 GCs connected in parallel to one autosampler...

[PeterB]

I'm using an Entech 7032 gas autosampler to fill a large sample loop which includes four gas valves and four GC channels in two 7890 GCs. I'd like the autosampler to wait for both GCs to be ready before it fills the loop, and I'd like it to then start both GCs.
I can convince both GCs to start together, but I can't convince the Autosampler to wait for both GCs to be ready.

Anyone have a cabling diagram for two GCs connected simultaneously to a single (third party) autosampler.
Anyone working with something similar to this configuration?

[Peter Apps]

No idea on the communications I'm afraid but as a workaround you can set one GC to have a cycle time that is longer than the other one needs to get ready, and then use the ready signal from the GC with the long cycle time.

Peter

[rb6banjo]

Going with Peter's line of thinking, what if you set the oven-equilibration time on one of the GC's to something that is long compared to what it takes to really get them both ready? Have your sampler go off the one with the long EQ time. This might be an easy way to do it.

The old Agilent systems have that EQ time feature. I don't have a 7890 but I assume that you can still do it.

[James_Ball]

To make it work with cabling you need to know what pins are the Ready signals and if they are open or closed when ready.

If it takes and Closed signal to be ready then it should work, as you wire them in series and once both instruments close their relay you then will get a full loop for the current flow to signal both are ready.

If it takes an Open signal, that would not be so easy since once one opens the circuit the current flow stops and the autosampler thinks everything is ready.

[PeterB]

Pin 1 (on the autosampler end) is the GC ready line. Voltages read quite low on it at 0mV when ready and -70mV when not ready.

Our tech guys created a possible work around cable with two GC ends, and it works with each GC individually but not with both together. Even with the individual GCs though, the voltages through our custom read significantly differently to the Agilent Cable (+70mV ready, 0mV not ready with one connected; with both connected ~+75mV neither ready, ~+105mV one ready, ~+145mV both ready), and I suspect that the two diodes in the ready line of the Agilent cable are changing the signal in a way I might not be able to reproduce.

So was wondering if there was a ready made solution out there, but maybe out of luck. At worst, I can push the autosampler runtime out past the longer GC run so tht the ready signal isn't an issue, but it would be nice to have it working properlhy!

Thanks to those who provided input!

[Peter Apps]

It is a very long time since I worked on Agilents, but I recall that they signal using 5V pulses rather than open or closed circuits. If this is so you have to have a way of making the autosampler wait for the second pulse, which I suspect is not going to be easy.

Peter

[James_Ball]

I seem to remember when using an Archon autosampler for volatiles with the 6890 and up there is a resister in the line to increase the voltage from the ready or not ready signal.

From what you described though it sounds like it is giving an open for ready which would be a problem.

For that situation you will probably need a logic interface that would take the signals from the two GCs and combine them so that it sends the open signal to the autosampler once the two are ready. This would have the two GCs not directly connected to the autosampler, but have and interpreter so to speak between them. Once two open signals are received to the interpreter box from the GCs it then sends an open signal to the autosampler. The autosampler then most likely sends a pulse back to the GCs indicating an injection has occurred which is the Start signal for the GCs.

It is a little beyond my experience in electronics but anyone who can make simple TTL logic boards could probably make one up easily.

[siperst]

Could you help me with a manual for Entech 7032?
Sincerely,
Joseph
jsiperstein@ohiolumex.com