APPENDIX 7

CONTROLLING THE OUTGOING LINES--DTR, RTS & XMT

There are up to three outgoing RS-232-C signals on each of the RS232ports of the Interface Module. Data Terminal Ready (DTR), Request toSend (RTS), and Data Transmit (XMT). Each of these lines can beturned ON or OFF with the CONTROL command.

Port 1 supports all three outputs. Ports 2 and 3 have DTR and XMT;port 4 has only XMT. You may use this command the same way with any port--it is not an error to try to control a line that does not exist.

Your attempt will simply have no effect.

You may control any or all of these lines on a single RS-232-Cport with the CONTROL command (controlling lines on other ports requires one CONTROL command for each port). The CONTROL command may be issued to a port which is not OPEN through an I/O channel by specifying any unopen channel number in the CONTROL command. If the port has been opened through a channel, you must use that channel in the CONTROL command. You may not issue a CONTROL command if any concurrent mode I/O is active.

Controlling XMT line has very limited use and few users will be concerned with it. In its normal state XMT is passive. If you change XMT you are likely to interfere with the normal transmission of data. In the serial communication world the only practical use of control of the XMT line is to send a BREAK signal. The BREAK is simply a period

of holding the XMT line out of its normal resting state. Specifically, the normal resting state is called MARK, which corresponds to the binary "1" state, A BREAK is a period of the state called SPACE, which corresponds to binary "O". (Actually, since MARK and SPACE are the only legal states of any RS-232-C signal, all data consists of alternating MARKS and SPACES. What distinguishes BREAK from other uses of SPACE is that BREAK is a SPACE which is a lot longer in duration than the time that a transmitted word would be. This is so because any transmitted word ALWAYS has one or more MARK bits in it--in particular, each word ends with one or more stop bits represented by MARK. ) Thus to send a BREAK, first issue a CONTROL command to set the XMT line to SAPCE (O), then a little while later issue a control to set it back to MARK (1).

The uses of the other lines will depend on your application. For some guidelines, see APPENDIX 1.

The pre-set default state of the DIR and RTS lines is OFF. The pre-set default state of the XMT line is MARK. Once you change any of them with the CONTROL command, the new setting will remain until you either turn the computer off or issue another CONTROL command to change things. The SYSTEM RESET key has no effect on these lines.

The form of the CONTROL command in BASIC is:

XIO 34, #channel, Aux1, Aux2, "Rn: "