24 Jun
2013
24 Jun
'13
3:59 p.m.
On 6/24/2013 5:17 PM, Ethan Dicks wrote:
On Mon, Jun 24, 2013 at 5:36 PM, Jim Brain <brain
at jbrain.com> wrote:
I should probably not sound so authoritative, since I
am sure something
used the lines.
But, since I was intrigued, I popped open a VIC1101A RS232 cart, and it
pulls the + phase of the 9VAC, rectifies it via single diode, cleans it
up with a cap, and uses that for + on a 75188/75189 pair (1488/1489
compatible, I presume).
The (-) leg is more interesting. It looks like it is created on the
cart via a RC circuit, a B562C PNP and a small transformer from the 5V
supply.
Weird setup, since I thought the VIC1101A was from the VIC-20 era, where
the -9V would be easily available on the user port.
There's a story behind that.
Jim
In practice, no one uses the 9VAC on the 64 and
VIC as AC. It's not on all
user ports, so systems don't plan on it being there...
What about as a source
of negative voltage for the RS232 cartridge?
I haven't seen a schematic of it lately, but IIRC, it uses a 1488 and 1489
and needs a bipolar supply and I don't _think_ it has an on-board negative
supply fed by a charge pump.