3 ms·

Almost 20 years ago I worked for a company that was bidding on replacing the CRTs that had been rendering platform train information, which was spat out as a st

Almost 20 years ago I worked for a company that was bidding on replacing the CRTs that had been rendering platform train information, which was spat out as a stream from said PDP11. I even went to Flinders St and got to see it. It was everything you imagine a 70s era data stream to look like. 1200bps, weird control sequences, etc., etc. And no-one could really tell us much about it, but there was some poorly photocopied incomplete documentation.

SulphurCrested 2y ago
I worked on this system during its development in the 1980s. There were actually two PDP-11s, the one to run the platform displays running locally-written software. The “weird control sequences” sent between the PDP-11 and the plaform displays were HDLC, a synchronous protocol then common in IBM token ring networks as SDLC. This was actually a decent technical solution because they only had to run one coax cable down each train line and the PIDS could sit there watching for their token slot. The hardware for HDLC would have been commodity, whereas fibre optic or carrier sense for long-run packet was not. The other PDP-11 that ran the signals (the “train describer”) could plot the position of trains on glass TTY terminals using escape sequences (VT100, same as in xterm today) so our PDP-11 pretended to be one of those and screen-scraped. So I was told, when I asked the guy who wrote it. All of that was done before I got there. I was called in with 6 weeks to go before commissioning to fix the bit in the middle that recalculated train positions and arrival times.
- aryonoco 2y ago
  As a Melburnian and a PDP-11 bits collector, this is just fantastic information! Do you have a blog or somewhere else where you share tidbits of information like this? Cause I sure would love to read that.

3 ms·

Almost 20 years ago I worked for a company that was bidding on replacing the CRTs that had been rendering platform train information, which was spat out as a st

by FireBeyond 2y ago

SulphurCrested 2y ago
I worked on this system during its development in the 1980s. There were actually two PDP-11s, the one to run the platform displays running locally-written software. The “weird control sequences” sent between the PDP-11 and the plaform displays were HDLC, a synchronous protocol then common in IBM token ring networks as SDLC. This was actually a decent technical solution because they only had to run one coax cable down each train line and the PIDS could sit there watching for their token slot. The hardware for HDLC would have been commodity, whereas fibre optic or carrier sense for long-run packet was not. The other PDP-11 that ran the signals (the “train describer”) could plot the position of trains on glass TTY terminals using escape sequences (VT100, same as in xterm today) so our PDP-11 pretended to be one of those and screen-scraped. So I was told, when I asked the guy who wrote it. All of that was done before I got there. I was called in with 6 weeks to go before commissioning to fix the bit in the middle that recalculated train positions and arrival times.
- aryonoco 2y ago
  As a Melburnian and a PDP-11 bits collector, this is just fantastic information! Do you have a blog or somewhere else where you share tidbits of information like this? Cause I sure would love to read that.