The Z38xx and similar SCPI receivers are a special case. Their time code message is specifically synchronized to the 1PPS. If you enter the :PTIM:TCOD? command to get the time message, the receiver "blocks" for up to a second until the 1PPS time. They also have a second serial port dedicated just to the time code message. In the Symmeticom UCCM-P receivers the time code will even come out in the middle of things like the SYST:STAT? message or while the user is typing a command. The Trimble UCCM receivers will skip sending the time code message if cannot be sent due to other output going on at the tome or the user is typing a command. Most timing receivers also carefully control when the time message is sent, particularly when operating in binary message mode. --------------------------- >In case anybody is concerned about the PC end of serial timing, I have good data from a Z3801A feeding Linux. With ntpd's filtering, it's well under a ms.