I had those in the 747 Classic. Most of them worked in Standby Mode when I was off the flight deck, especially if the FO was asleep. However, they tended to lapse into sleep mode in cruise...
We were happy campers when we climbed on board to discover that the aircraft had PMS. It was kind of weird, though, to see it climb or descend +/- 100' from the MCP altitude during cruise.