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Modified Datum 9300 Introduction - The HP Z3801A produces two differential PECL 1 PPS outputs on its J3 connector. Either can be used to sync the Datum 9300. A word of warning... There seem to be a vast number of different Datum time code readers called "9300" and they are all completely different. I have a number of new units, still in their sealed factory cartons. Contact me if interested, or if you need more details... Datum 9300 Circuit Description - The unmodified 9300 divides the 60 Hz line frequency down to 1 PPS and uses that for sync. The modifications to the unit will allow the precision 1 PPS pulses from the Z3801A to be substituted. The first order of business is to get the signal to the 9300. Fortunately there are unused pins on its input connector. Use a shielded, twisted pair and bring both sides of the differential PECL signal as well as ground to the 9300. The 9300's internal circuitry is floating and it will need to be referenced to the Z3801A's signal ground as I have shown. Next we need to convert the signal to a single-ended TTL level so it will be compatible with the TTL circuitry in the 9300. I chose the On Semiconductor MC100ELT23 for that job, but anything that works is OK. Datum 9300 Modifications - We'll need to disconnect the 60 Hz derived 1 PPS signal and replace it with the new one - without harming any other functions. To do this we need to disconnect both inputs of the U23 NAND gate shown in the block diagram. The top input is the old 1 PPS signal (pin-1). Whenever the mode switch is moved from "LOAD" to "RUN" a single pulse is fed to the bottom input (pin-2). This forces the clock to start "ticking" the instant the switch is moved. That's important if you're setting it by hand, but we don't want that. We want to pick up the next GPS tick, so that signal is no longer wanted. Cut those inputs (red "X") and tie both inputs together and connect them to the output of the PECL-to-TTL translator's output as you see in the block diagram. Operation - Select "LOAD" on the mode switch and load in the day, hour, and minutes with the thumbwheel switches. Wait for the correct time to arrive and then select "RUN" within one second. The clock will pick up the count at that point and stay synchronized to the GPS clock from then on. Now take the IRIG-E output of that clock and feed it to other 9300's or use it as it was intended and feed it to one channel of a recorder and log the day and time! |