Like most IBM mainframe devices the IBM 3270 terminal offers a rich set of functionality to the programmer. The problem with a lot of functionality is the corresponding complexity. I will attempt to break down the complex data stream so that it can be more easily understood. For now I will only concentrate on the basic 3270 model 2 that ruled the world when MVS 3.8 was alive and well. I will also focus on the basics for now but reserve that right to add some of the advanced functionality at a later date
The 3270 expects to see the following data presented in the proper sequence:
The Write Control Character (WCC) is a one byte command that is always the first byte of the data stream sent to the 3270. It has the following format:
| * | Reset |
Printout Format |
Start |
Sound Alarm |
Keyboard Restore |
Reset MDT |
|
0 |
1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Bit | Meaning | |
| 0 | The value of this bit is determined by the value of the other seven bits | |
| 1 | Reset | |
| 2 | Bits 2, 3 and 4 are used for controlling printing | |
| 3 | since we are not concerned with printing right now we can | |
| 4 | ignore these bits and always set them to zero | |
| 5 | Will sound the alarm (make the terminal beep) | |
| 6 | Unlocks the keyboard | |
| 7 | Resets the Modified Data Tag bit for all input bits |
A very common WCC to use is x'C3' or B'11000011'. This WCC instructs the terminal to unlock the keyboard and clear the MDT for all input fields.