Plotter Interface

HP7475A Plotter

The HP plotter was available with various interfaces, mine has a 25 pin female D-connector. This is an RS232 interface that requires a handshake. This has led to some minor problems, as HP use the data terminal ready (DTR) control line to signal that the plotter is ready to receive data or not. Different operating systems have a greater or lessor implementation of flow control/ready-notready. The cable requires a cross-over as both sides of the link are setup as a DTE. The other linked pins at the computer end indicate that there is a device connected, hopefully. The connection as shown will work on MS-DOS and Windows upto WINXP.

PLOTTER                            COMPUTER
    25-pin                         9-pin                   25-pin
    7  0volt                        5 0volt                 7 0volt
    2 RX                            2 TX                    3 TX
    3 TX                            3 RX                    2 RX
   20 DTR                           8  CTS                  5 CTS
                                    7 to 6 to 1             4 RTS to 6 DSR to 8 DCD

As can be seen from the cable wiring the plotter uses DTR as the handshake, CTS being 'clear to send', to pause data flow across the interface. You basically cannot use a standard cable, it should be wired as shown above.

It is now November 2015 and with a faster system the DOS software is proving to be a little touchy. Although the edit program runs OK the plotter software is very tempremental, failing to reset the plotter, then looping.

There are two methods that maybe used to 'get around' this problem. One is to use the Windows version of Autotrax to plot. As mentioned previously there is a free version, but it does not allow saving to a file. However, it will read files produced by the Dos Autotrax Edit and the Windows version can drive the plotter. The plotter has to be installed in Windows with a suitable driver.(see below)

The second method is to use the Dos Autotrax Plot program but to write the output to a file. The file will consist of command sequences of ASCII characters conforming to the HPGL protocol. This file can be sent to the plotter using a terminal emulator such as Hyperterm. Both methods work but it is easier to put a non-standard pen into the pen holder using the latter method.

The Windows version of Autotrax will automatically load a pen from the carousel and continue to plot which is fine if the ink in the pen is waterproof. Also a Windows driver is required, I found the drivers for Roland devices work with the HP7475A with WinXP.

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