Yes, you're right. I just took a quick look at the T200 schematic and saw few gates sitting at the RS232 connector, I didn't notice the VEE at the bottom (plus the image is very pixelated too).

On 8/9/2017 4:20 PM, Mike Stein wrote:
I think you're right about the drive side signals being at TTL levels and not 
'true' RS232, but if you have another look at the T200 schematic (or actually 
measure) I think you'll find that like the other model Ts it does indeed 
conform to the RS232 spec with a swing of -5V to +5V.

I believe the 'special cable' takes care of the necessary translation.


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Sent: Wednesday, August 09, 2017 1:52 PM
Subject: Re: [M100] PDD Firmware Reverse Engineering - Progress

There should be a little difference in speed, the data to and from the
disk in not streamed, it is buffered in RAM in chunks on the drive
controller board and then transferred to disk (or back to serial). And
everything is done asynchronously.
I'm sure 19200 was chosen for robustness though, the RS232 interface on
the drive controller is not a proper implementation and only provides 0
to 5V signaling for "mark" and "space", not the proper -3V to -15V for a
"mark" and +3V to +15V for a "space". And looking at the schematic for
the T200, it's the same way too.


On 8/9/2017 1:27 PM, John R. Hogerhuis wrote:
Interesting! I wonder if it can actually do 76800bps since it's in
their baud rate table.  I've found that's the top speed of the model
100. I don't think the t200 can reach it though. I think the t200 will
function at 38400 but I don't remember for sure.

Not that it matters too much. It's a disk drive so we're limited by
the speed of the drive itself.

-- John.

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