Mike's PCjr Page

I've seen somewhere there was a protoboard for the modem slot.

I plan on writing a short doc on the pitfalls of the Modem slot and the Back side serial port.
As many probably know the Jr is not consistent with the Standard PC bios and handles these rather weirdly.
Any way I will put together a short 1 or 2 page info on this..
I will also do a write up to add RING Indication input to the Back serial port That is missing.

In the meantime I need to make a plug in serial port for the modem slot, to have this as a true serial port
and wanted to get my hands on one of these proto boards./

If you know the person that made this or something similar let me know, I and about 1 day away from just spinning a board
with all the parts on it for the serial port, making it a short card so it does not interfere with my 3FlopBrd that kinda blocks
the back side of the modem slot....

Do you mean this?

https://www.brutman.com/PCjr/pcjr_jrcombo_card.html

No it was just a basic proto board, not dedicated to any function...
But I may have convinced myself that spinning a real board with SMT is the right way to go.....
I will work on it this week and decide if I want to hand wire one board or just quickly spin some PCBs with a full blown serial port function available..

Like this then: https://www.ebay.com/itm/IBM-PCjr-Modem ... 4389322040

there we go...
That might work, but I think I want to evaluate the SMT version, I can get it smaller.
Will try a quick layout this week....
thanks

Spent most of the day laying out a plug in serial card for the modem slot,
Kept all the parts SMT, and at 5V levels.
this will give the JR a functional RI Ring indicator input for external modems. this is missing on the Back side Serial Port..

Using the system supplied baud freq. 3.579Mhz.scaled down to 1.7xxx this still limits the usable Baud to,4800, and looking at the Bios seems to be the limits there also.
(if someone has info that is contrary pls share, as,I've not done an indepth review)

But figured that for an external modem 2400 Baud would be ok, keeping the system as close to the original design specs is ok with me....

so Hope to have the board done this week and then get a few in to test...

just an update...

What UART are you using?

Faster speeds would work with a FIFO, like the 16550. But yes, the BIOS is limited. And software can be used to set the divisors, but most software assumes the PC timings and not the PCjr timings so at faster speeds they diverge too much, requiring PCjr specific software to set the divisors.

Unless of course you put your own oscillator/clock on the board to get us to the same PC divisors ...

Yea I thought of the on board crystal, but the Bios was the concern, so I may put the footprint down and play with it as a separate project...
thanks Mike

monzamess wrote:Like this then: https://www.ebay.com/itm/IBM-PCjr-Modem ... 4389322040

sorry folks i was traveling for work and missed this post! that is my prototype board design! i still have a few left if anyone is interested! i'm also open to requests for new designs!

Dave

IMO a 16550 with an external crystal that can do 9600 baud or better is more suited for something that occupies the card slot and I/O port 3F8. I assume you'd have to use software that directly used the I/O port like a PC, though, and not through the BIOS.