Hi - came across your query while looking for something else and it caught my eye because I used to be an expert - around 30-35 years ago! Regret I have not been able to find the appropriate spec anywhere but fwiw -
The 3278 coax level protocol was very different from the 3277 in that the screen buffer was managed by the control unit, which polls the terminal for keystrokes and echos the characters to the terminal. (The 3277 managed everything at the terminal until the user pressed an attention key, when the control unit would read the whole screen buffer.) The protocol is very simple, as the control unit does almost all the work, including translating keyboard scan codes to characters for different character sets, and managing the cursor position. As far as I remember almost everything was achieved with 3 commands - POLL, SET ADDRESS, and WRITE. The character set was proprietary and different from the 3277. So you could maybe figure that out by sniffing a wire, you would have a lot of work to get from there to 3270 data stream level.
We built a pc card to emulate the control unit using the National Semi DP8340/41 chip set. The spec here ftp://bitsavers.informatik.uni-stut...ations_Local_Area_Networks_UARTs_Handbook.pdf
gives some detail of the physical interface for the coax but unfortunately nothing about the protocol. (you need to go down about 170 pages to find the relevant bit)
Good luck!
The 3278 coax level protocol was very different from the 3277 in that the screen buffer was managed by the control unit, which polls the terminal for keystrokes and echos the characters to the terminal. (The 3277 managed everything at the terminal until the user pressed an attention key, when the control unit would read the whole screen buffer.) The protocol is very simple, as the control unit does almost all the work, including translating keyboard scan codes to characters for different character sets, and managing the cursor position. As far as I remember almost everything was achieved with 3 commands - POLL, SET ADDRESS, and WRITE. The character set was proprietary and different from the 3277. So you could maybe figure that out by sniffing a wire, you would have a lot of work to get from there to 3270 data stream level.
We built a pc card to emulate the control unit using the National Semi DP8340/41 chip set. The spec here ftp://bitsavers.informatik.uni-stut...ations_Local_Area_Networks_UARTs_Handbook.pdf
gives some detail of the physical interface for the coax but unfortunately nothing about the protocol. (you need to go down about 170 pages to find the relevant bit)
Good luck!