Attached are a few plots of bits right on the RS232 line, for various gate times. First plot, "HP53131A_BAUD.png" serial transmit duration ~10.4 ms for 20 characters, plus one start and one stop bit yields a baud rate of 19.2 kBaud, as expected. The next plot, "HP53131A_1sTrigDelay_1sGate.png" is just as wonderful as the many NEMA serial debates, the mean period is ~1150 ms, with +-50 ms of jitter. The next plot, "HP53131A_100msGate.png" shows the measurement rate is no where near 10 Sa/s, but is approximately 4 Sa/s. The serial transmit duration is only ~10ms so its not being bandwidth limited on the UART. Again roughly +- 50 ms of jitter. Finally plot, "HP53131A_10msGate.png" the measurement rate is only 6.25 Sa/s for a 10 ms gate time. The jitters is slightly better at +- 25 ms. For time interval measurements the 53131 keeps up to about 17 events/s before it starts dropping samples. When I pick up a long enough GPIB cable I will see how much better the results are over GPIB versus RS232.