5v on pin 18

[flip]

Hi,

I've voided my warranty already: to get a serial to parallel adapter for the C64 working, it needed 5v on pin 18 of the IEEE connector. I've soldered a wire from the diode to pin 18, but it could be something to consider for the next H/W revisions... I don't think it's part of the standard, but some specs mention the possibility of having +5v provided via this pin...

Philip

[RWAP]

Pin 18 of the centronics port - normally unused - I wonder how they got printers which worked with that (or was it the Commodore branded printers)?

Unusual - we have come across 5V on pin 13 and on pin 32 but not pin 18!

I guess when we come up with the scheme to be able to select whether there is 5V on pin 13 or pin 32, we could add the ability to supply 5V to pin 18 too!

[flip]

Some specs show 5v on pin 18 - https://goo.gl/images/9sym83

Other have it as "host logic high" - https://goo.gl/images/T5oGjB

And i've seen a max draw of 50mA somewhere, but no reason it couldn't be more...

Philip

[flip]

or was it the Commodore branded printers

Could be wrong, but I think commodore printers all used a serial port?

Philip

[RWAP]

I think I have a commodore branded one in the loft, but can't remember what connector is on it (I never owned a Commodore machine).

I wonder what pin 18 was actually being used for - it didn't appear to be required on any of the industrial machines and computers we tested the retro-printer with, but then none of us had a C64!

[flip]

more google-ing revealed this (from http://nemesis.lonestar.org/reference/c ... onics.html):

VCC (aka SLCT)
On Centronics-compatible printers, pin 18 was connected directly to a +5 VDC source within the printer. The host interface adapter could utilize this source of power to hold signal lines in reasonable states when the host computer system was not turned on but the printer was. The printer typically specified that the host could draw no more than 50ma from this line, but may not actually have circuitry to prevent excessive current damage. This +5 VDC was present whenever the printer was turned on.

For various reasons, some Centronics-compatible host systems tried to use this signal instead to indicate the presence of a printer. If power was present, a printer was present, even if it was not necessarily available at this moment. This thinking is somewhat flawed, because the host operating system really could not behave any differently based on this information. The computer would still have to check for a printer in case one was turned-on or connected after the operating system had made its initial check, since no one would accept a system where you had to shut-down and restart a mainframe computer to which a printer was attached, just because the printer wasn't turned on first the last time the mainframe was started.

The IBM-PC uses the signal on pin 18 of the printer connector as "SLCT", which is supposed to indicate that the printer is selected. However, if you examine the schematics of the typical printer, you will find that the printer simply places +5 on pin 18 at all times (per the Centronics standard), so on IBM PC systems this signal doesn't indicate if the printer is selected or not, it merely indicates that the printer has power. Unlike other signals on the IBM PC printer interface, this signal is not pulled-high, so it should accurately reflect whether electricity is flowing from a connected printer or not.