Hotplug LCD display?

[mkutter]

Hi,

is it possible to hot-plug the LCD display (i.e. plug/unplug a LCD while the core is running)?

I'm currently building a MIDI switcher/arpeggiator, which is designed to sit in the back of my rack and will not have any controls, but I'm thinking about building some kind of remote control containing a 2x16 display, one encoder and 6 push buttons, connected via a 25 pin cable (standard parallell port cable w/full wiring).

I've already found from another thread that the DIN lines basically are hot-pluggable and wondered whether I could hot plug the LCD, too.

[Screaming_Rabbit]

... think about the cable length!

Greets, Roger

[mkutter]

Hi Roger,

I don't think that will be an issue - a 5m 25pin D-Sub SCSI cable has an around 15 Ohm, so the voltage drop is not that big. And I guess SCSI is clocked as high as the MBPH LCD bus.

What I'm more afraid of electrically is that the MBPH core won't be able to handle the suddenly increased power consumption - and in term of MIOS, I don't know whether there's a way to initialize a "new found" LCD, and to de-initialize a unplugged one.

Greets,

Martin

[/tilted/]

Hiya.

voltage drop is only one concern here.

you also need to worry yourself with cable inductance and capacitance, both of which act as a kind of 'low pass filter' for your LCD control signal. They will limit the current and voltage slew rates respectively, and this in turn means potentially lost data. The cable limitations can be overcome to an extent by driving the cable using extra hardware in the form of transmitter/reciever buffers.

Regarding the hot-plug issue, I'm not sure, but I think the LCD is initialised by the core at start-up. This coulld perhaps be overcome by using a spare D-sub pin to act as a plug detect pin, forcing the core to init/reset the LCD when the plug is inserted.

[mkutter]

Hi /tilted/,

thanks for your answer - using a spare D-Sub pin as plug detect is something I've been thinking about, too.

Greets,

Martin

[Wilba]

In MIOS, if you unplug the LCD, then subsequent commands to the LCD timeout (because they don't get a reply from the LCD) and it is disabled in MIOS - MIOS won't send it any more commands. You would need some code to reinitialize the LCD, and some  code in the application to know when you plug in and do a full display update at that time... knowing when you plug in can be done with a single grounded DIN input on your control surface, i.e. like a permanently pressed switch.

But is it really necessary to hotplug it? How important is keeping it running while you plug in the control surface?

[mkutter]

Hi Wilba,

thanks for the reply, I'll just try it out.

The reason for wanting it hot-pluggable is just laziness: I don't want to have to double-check I've switched the thing off before I plug in the control surface.

I'll post the results :-)

Greets,

MArtin