RS-232 Problem (Configuration ?)

From: Tony Duell <>
Date: Tue Feb 18 13:49:00 2003

> Let me start off by saying "I hate RS-232". Anyway, now that that's

I know the feeling. I do battle with it far too often, and it takes too
much of my time. No 2 manufacturers seem to use the signals the same way...

> out of the way, I am having a problem with an RS-232 interface on an
> IMSAI SIO board.
> I've been debugging a North Star drive and I was having problems with
> it booting (DOS 2) in my IMSAI. I would get the "*" prompt but OS

OK, I have the schematic in front of me.

> would not recognize my Soroc terminal input. At first I thought it
> might be buggy code on my DOS floppy.
> Anyway, to make a long story short, I have multiple of everything and
> have ruled out everything but something to do with my RS-232
> configuration (I guess). I have 2 SIO board and both behave the same
> way.
> I'm documenting what I have here, in case someone sees something
> wrong that I've overlooked. BTW, this was previously working without
> any different configuration or hardware, with the exception of the RS-
> 232 cable from the Soroc to the IMSAI. This is a brand new cable
> that was working just a few weeks ago so I'm having a hard time
> believing there is anything wrong with this cable. I have only one,
> if you can believe that. I'm going to get another this weekend, if I
> have not figured this problem out by then.
> Anyway...
> I'm using Port A (on the SIO) and have configured the A3 Jumper to
> have
> TD Pin 1 to Pin 15 RD
> RD Pin 2 to Pin 16 TD
> RTS Pin 4 to Pin 12 CTS
> CTS Pin 5 to Pin 13 RTS
> DTR Pin 7 to Pin 9 DSR
> DSR Pin 8 to Pin 10 DTR

OK, that seems like a possible configuration. The schematic shows that
pins 9, 10, 12, 13, 15 ,16 go to pins on the cable connector, so the
correct wiring of this jumper block depends on the cable.


> 1) Is the A3 header correct for the IMSAI as the computer end and the
> Soroc as the terminal. 3 pairs crossed over. It does write to the
> Terminal properly and when I press the Break Key, I get an echo from
> the IMSAI of *?, which seems normal.

The fact that you get valid output from the Imsai to the terminal
suggests that you've not got the connections crossed over (i.e the data
output at one end is going to the input at the other).

What have you done with pin 3 of the A3 jumper? Ift surely should be tied
low (disabling the current loop circuit), or the '08 gate at C4a will be
inhibited. This will prevent data input.

> Anyone see something bonehead that I overlooked?

Waht I would do at this point is look at the RxD and handshake input pins
at the 8251 USART at B5 on the board.See if you can get a data stream at
the RxD pin, and that the handshake lines are asserted (low). If not,
then you know where to start looking.

Received on Tue Feb 18 2003 - 13:49:00 GMT

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