Terminals switch to BLK Communication mode.

[Eric Malzahn]

We're running a new application on Wyse terminals in VT100 mode. Once in a while the terminals will toggle to BLK communication mode from FDX freezing the users screen. This is easy to fix, but I'm not sure why it is happening. Can anyone explain BLK communication and why an application would cause terminals to switch to it? Thanks...

[Mark Fenton]

Eric, man a blast from the past!
Can't vouch for Wyse terminals, but where I've seen the term is with older mainframe attached terminals. BLK = Block mode and is a communication mode where local input is buffered and sent in blocks to the mainframe. Last computer system I used that had that was an old navy firecontrol computer that a certain nameless government agency had parked in a facility somewhere outside of the US. There were some number of dumb terminals directly connected to the mainframe and block mode transmission was used in certain screens of the database program to minimize the number of interrupts the computer had to handle.

Toggling back and forth between stream and block mode was handled by the database program itself.

A later version of this setup with semi-intelligent terminals (they actually had their own built-in text editors!) still had the ability to switch between block and stream communication mode, but then it was handled by an escape sequence from the keyboard.

This was all back in the early 80's, so I won't pretend that I remember all the details clearly.

hth (and happy delving into history...)
Mark

[Eric Malzahn]

Thanks for the information. I think the onus is on our app developers here, but I wish I knew what the command is (escape sequence or control character) that's doing it.

Thanks again!
Eric

[A. Clay Stephenson]

This is an oldie question. Most Wyse terminals went into block mode (i.e. the terminal does local editing and then transmits an entire block of characters to the host) when it received an 'ESC B' sequence. ESC B is close to ESC [ B the cursor down vt100 sequence. If you have a handshaking problem it is possible the '[' is missed. The other thing that will cause really strange terminal problems is running with the kernel tunable 'timeslice' set to 1 rather than the desired value 10. This is especially true on recognizing multiple character input sequences like the 'arrow' keys.