IC-7300 “SEND” signal – Be Careful when using it

There has been an ongoing discussion for quite some time about a reported “RF Tail” on the IC-7300. This report is based upon the fact that the 7300 provides a SEND signal that is supposed to be useful for telling offboard equipment if the 7300 is currently Transmitting or Receiving. Actually there are two of these signals, one is connected through the “ACC port” DIN connector. The other can be found on a rear-panel phono (or RCA) jack.

It has been shown that these signals do not always accurately indicate if the 7300 is currently Transmitting RF, as you might think it should. You can search and find all the detailed data, but the big concern most people have been discussing is the “RF Tail,” at the end of a transmission, with the CW case being used as an example. In this case, the user stops sending a character and the SEND line returns to the Receive State, while the RF decays and eventually ends, roughly 2-3 milliseconds later. This is certainly not what one would expect the SEND signal to do, either intuitively or from the Manual. One test also shows that the timing is different between the two SEND signals.

I have been following this discussion which has been off-and-on over many months. In the past two weeks, it hit a fever pitch, with all kinds of comments about it, test data being posted, and even some reaching a “final conclusion” that it is NOT an issue. I think we may want to slow down a bit, before claiming that it is not an issue. To me, determining if this is an issue depends upon several things:

  1. Is there a possibility of damage to the 7300 or interconnected equipment due to this signal NOT doing what is advertised and expected?
  2. Is there a weakness in the design?
  3. Is there a problem with documentation?

It appears to me that the nature of the most recent discussion was to dismiss this as a non-issue, and be done with it. After much discussion and many reports from users, it was proclaimed that “not a single Amplifier was damaged” and this was the basis of some saying it is thus a non-issue. I disagree, with the following thoughts on each of the above 3 potential areas of concern.

  1. Could damage to the 7300 or connected equipment result? – It is certainly possible, depending upon the interconnected equipment and switching times. In the public discussion, it was pointed out that relays used in Amplifiers are typically “slow” and they will likely take longer than this RF Tail to switch from Transmitting to Receiving paths. That may be likely, but I am not used to seeing any manufacturer specify the “minimum contact closing time.” The claims I have heard are based on a single measured closing time, or general expectations. Also, someone designing an outboard TX/RX switch today might very well be using PIN diodes specifically to get fast switching times.
  2. Is there a weakness in the Design? – YES. Surely the Designers did not intend to provide a “SEND” signal that transitions back to the inactive (RECEIVE) State 2-3 mS prior to the end of RF energy Transmission. Experiments have shown that other ICOM rigs with similar interfaces (I think the 7600 and 7610 were used as examples) do not have this “feature”
  3. Is there a problem with Documentation – Certainly the User Manual does not do a good job of explaining the timing of these signals. Vague terms are used like “for use with non-Icom Amplifiers” and “the pin goes low when the transceiver Transmits.” I have not seen the Service manual, so I can not comment on that, but most Users probably do not have the Service Manual. As a System Engineer, would I be satisfied with the documentation I have seen, in designing an Amplifier interface using these signals? – NO –

In summary – the IC-7300 clearly has an anomaly which results in the “SEND” signal sometimes failing to correctly indicate the current TX/RX State, at least as far as the presence of RF energy on the Antenna connector is concerned. This seems like something that should be corrected to me. At the least, it should be clearly documented in the User Manual, with some worst-case timing diagrams.

I don’t think relying on Amplifier relays being “slow” is a great solution to this issue. I have heard that some HF Amplifiers have a setting that can be used to accommodate these types of timing issues. I have used my Elecraft KPA-500 Amplifier with my 7300 a few times, with apparently no problem. I will surely look at any KEY-timing related settings before I do that again.


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