Trying to ID some hardware - RS232 Interface

[bobkat2769]

Hi there,

 

I've got a piece of hardware that I'm trying to ID and could use some help.

 

I do know what it is, or is supposed to be. The case is labeled "RS232 Interface" and it has an SIO jack at one end and a DC power jack plus a DB-25 serial jack at the other end.

 

While the case is definitely very homebrew, the double-sided circuit board inside looks very commercial and has "SKFF 27 rev 3" printed on it along with a sort of pyramid shaped "SG" logo (S for peak, G for bottom) as near as I can make it out.

 

There's really not much to it. Two resistors, two diodes, two 100uF electrolytics, and two 14-pin ICs. The first one's a MC1489N DTL Quad Line Driver. The other I've no idea as the markings appear to have been sanded off. Maybe a PAL, no telling.

 

Now, here's the weird part. I really don't see what good it would be as there just aren't enough wires going to the serial port, and one of them just doesn't make much sense to me.

 

Pins 2 and 3 are hooked up (TD and RD) which makes sense, but the last wire is hooked up to pin 7 (SG - signal ground) and it is hooked up to ground on the board but pin 1 - FG (frame ground is not).

 

There aren't nearly enough pins in use for a modem, much less a dumb terminal. But it's only short one wire for the dumb terminal (FG) which could have been possibly combined at the other end of the cable but I don't know if that would still work.

 

I do seem to recall that there was a reason for the seperate ground pins, but at this point in time I sure can't recall what it was. =^.^=;;

 

I'm sure it probably needs some kind of special driver. Anyone out there have any idea what it was supposed to hook up to and who made it?

 

Thanks much in advance,

 

BobKat =^.^=

[Rybags]

RS-232 can work in software flow control, and as such require a minimum of signals.

 

What you have there is probably some sort of interface/convertor for RS-232.

 

Or maybe even an earlier verions of the APE/SIO2PC interface.

 

Or, maybe the one which was designed early on to interface to the ST - wasn't there one called Rabbit or something?

[bobkat2769]

RS-232 can work in software flow control, and as such require a minimum of signals.

 

What you have there is probably some sort of interface/convertor for RS-232.

 

Or maybe even an earlier verions of the APE/SIO2PC interface.

 

Or, maybe the one which was designed early on to interface to the ST - wasn't there one called Rabbit or something?

 

With a dumb terminal, yes it could if both signal & frame ground were combined at the other end. But as I said before, I'm not sure that would work. I'm pretty sure there was a reason those grounds were seperate. Anyway, that would be the minimum required. But from what I remember for a modem or printer RTS/CTS are definitely required for anything beyond 300 bps unless you want a lot of framing problems, dropped characters or overflows. I've certainly never seen a serial modem or printer in my 25+ years of computing/repairing that used less than a minimum of 5 wires.

 

But looking at the article on RS-232 on wikipedia it seems there was a 3-wire version, but for flow control RTS/CTS wires are needed. I guess with a 300 bps modem you could get away with no RTS/CTS and use software XON/XOFF flow control as long as the modem had a large enough FIFO. Can anyone out there confirm this that has some experience with older serial devices like this?

 

Anyway, I'm pretty sure this isn't any kind of SIO2PC interface. First of all, this is late 70's early 80's era equipment. I'm not exactly sure how old this device is, it was given to me by a friend back around 1986 or so along with an Atari 800, 1050 drive, and a centronics printer interface. The date code on the line receiver IC is July 1979.

 

Secondly, the circuit isn't anywhere near complex enough to be an SIO2PC which probably didn't even exist back then. I certainly hadn't heard about anything even remotely resembling SIO2PC back then or I would have gotten one. And it's definitely not to interface with an ST. The ST wasn't even a pipe dream back when this sucker was built. =^.^=;;

 

Speaking of pipes and the ST, that reminds me of the Dobbs Head hidden in the ST character set! Ah, the joys of a bouncing Dobbs Head! =^.^=

 

Anyway, thanks for trying. But I already knew pretty much what it was since it IS labeled "RS-232 Interface."

 

What I'm trying to find out is what it was designed to be used with and what, if any, software I need in order to actually put it to use. I guess I should have been more clear about that. Sorry. =^.^=;;

 

Laterz,

 

BobKat =^.^=

[classics]

What you have sounds like an R-Verter interface.

 

Steve

[Rybags]

Shield/chassis ground would be seperate since chassis ground tends to mean the ground for the entire device (such as a PC case etc).

 

I'm guessing that sudden changes in potential would be a cause of interference, and maybe even damage/destroy controller ICs.

[kevtronics]

But looking at the article on RS-232 on wikipedia it seems there was a 3-wire version, but for flow control RTS/CTS wires are needed. I guess with a 300 bps modem you could get away with no RTS/CTS and use software XON/XOFF flow control as long as the modem had a large enough FIFO. Can anyone out there confirm this that has some experience with older serial devices like this?

I remember using LapLink to transfer files between two PCs in the '80s using their serial ports. You could use either a 3-wire cable or a 9-wire cable. The 3-wire cable worked fine as long as both computers were set to the same baud rate and had the same serial port UART chip.

[bobkat2769]

What you have sounds like an R-Verter interface.

 

Steve

 

I think that's probably what it is. That sounds about right.

 

Does anyone here know where I can find any info, preferably with a schematic, of the ORIGINAL R:verter homebrew interface? All I seem to be able to find is info on the newer version which isn't too much help for me.

 

I was able to find something on a much newer version using a MAX232 chip but that isn't what I have. The newer version uses the full 9-wire interface and not the very basic 3-wire interface I've got here.

 

I was able to find a device handler that is supposed to work with it. But I really don't have anything around I can test it with. I have plenty of old modems that still work (even a 300bps) but they're all ISA/PCI cards. The only external modem I still have is an ISDN modem that I'm pretty certain wouldn't do anything on a 3-wire interface as it connects to the serial port at 115.2 Kbps and really needs hardware flow control. =^.^=;;

 

BTW, it's a 3com ISDN Impact IQ. If anyone around here can actually use it make me an offer. So far as I know it still works as I've had it packed in the original box since I first got DSL about 10 years ago. If anyone's interested in it I'll hook it up first and make sure it can at least pass the internal self-tests.

 

One more thing, I'm a bit new here and have a very newbie question. =^.^=;;

 

I'm noticing the various sigs listing hardware and such, is there maybe some kind of FAQ somewhere with guidlines/customs for sigs and other newbie stuff around here that I can read to help me get oriented?

 

Thanks much for all the help so far,

 

BobKat =^.^=

[bobkat2769]

But looking at the article on RS-232 on wikipedia it seems there was a 3-wire version, but for flow control RTS/CTS wires are needed. I guess with a 300 bps modem you could get away with no RTS/CTS and use software XON/XOFF flow control as long as the modem had a large enough FIFO. Can anyone out there confirm this that has some experience with older serial devices like this?

I remember using LapLink to transfer files between two PCs in the '80s using their serial ports. You could use either a 3-wire cable or a 9-wire cable. The 3-wire cable worked fine as long as both computers were set to the same baud rate and had the same serial port UART chip.

I remember using laplink for several jobs xfering large amounts of data on clients' computers in the early 90s. At the time I couldn't believe people would pay me just to do that for them as slow as it was. But it was a lot faster than floppies and sneakernet and most small businesses didn't have networks back then.

 

I used a full 9-wire cable though. I never tried using a 3-wire cable. Especially since I had heard far too many horror stories from friends about data loss/corruption and/or low speed when they had tried it on their own stuff in an attempt to be cheap.

 

Laplink used to be a pretty indispensible part of my software tool kit for many years. I still keep a copy of it in my DOS disk pack. It's about the only disk in there that I haven't used in many years. Other stuff in there like Checkit and some of my DOS boot disks I still find handy for testing basic hardware problems in a hurry.

 

BobKat =^.^=

[dinosaur]

What you have sounds like an R-Verter interface.

 

Steve

 

I think that's probably what it is. That sounds about right.

 

Does anyone here know where I can find any info, preferably with a schematic, of the ORIGINAL R:verter homebrew interface? All I seem to be able to find is info on the newer version which isn't too much help for me.

 

Here is an Rverter reference:

 

http://atrey.karlin.mff.cuni.cz/~pavel/atari/rvert.html

 

I have the.zip file with the text and driver files,if you want it

Edited March 26, 2008 by dinosaur

[bobkat2769]

What you have sounds like an R-Verter interface.

Steve

I think that's probably what it is. That sounds about right.

 

Does anyone here know where I can find any info, preferably with a schematic, of the ORIGINAL R:verter homebrew interface? All I seem to be able to find is info on the newer version which isn't too much help for me.

Here is an Rverter reference:

 

http://atrey.karlin.mff.cuni.cz/~pavel/atari/rvert.html

 

I have the.zip file with the text and driver files,if you want it

Sorry, but that's not the one I'm looking for. That's the newer version that I already found before. Mine has a different design that's much older and simpler and only uses a 3-wire interface.

 

Thanks for trying though,

 

BobKat

[Rybags]

Hey bobkat.

 

Where did you get your av image from? Do you have a larger version I could look at?

[sup8pdct]

Hi there,

 

I've got a piece of hardware that I'm trying to ID and could use some help.

 

I do know what it is, or is supposed to be. The case is labeled "RS232 Interface" and it has an SIO jack at one end and a DC power jack plus a DB-25 serial jack at the other end.

 

While the case is definitely very homebrew, the double-sided circuit board inside looks very commercial and has "SKFF 27 rev 3" printed on it along with a sort of pyramid shaped "SG" logo (S for peak, G for bottom) as near as I can make it out.

 

There's really not much to it. Two resistors, two diodes, two 100uF electrolytics, and two 14-pin ICs. The first one's a MC1489N DTL Quad Line Driver. The other I've no idea as the markings appear to have been sanded off. Maybe a PAL, no telling.

 

Now, here's the weird part. I really don't see what good it would be as there just aren't enough wires going to the serial port, and one of them just doesn't make much sense to me.

 

Pins 2 and 3 are hooked up (TD and RD) which makes sense, but the last wire is hooked up to pin 7 (SG - signal ground) and it is hooked up to ground on the board but pin 1 - FG (frame ground is not).

 

There aren't nearly enough pins in use for a modem, much less a dumb terminal. But it's only short one wire for the dumb terminal (FG) which could have been possibly combined at the other end of the cable but I don't know if that would still work.

 

I do seem to recall that there was a reason for the seperate ground pins, but at this point in time I sure can't recall what it was. =^.^=;;

 

I'm sure it probably needs some kind of special driver. Anyone out there have any idea what it was supposed to hook up to and who made it?

 

Thanks much in advance,

 

BobKat =^.^=

 

here in australia, we had a terminal program written by john nicholls called mega term. it had a simillar interface to what you have except it was built into the case of a 25 pin shell serial plug. The chips are a 74ls126 and the 1489. this interface also has one of the rs232 signals going to one of the interupt lines. Motor control is used to enable the gates on the 126. your interface could be similar and could use the rverter driver.

james

[bobkat2769]

Hey bobkat.

 

Where did you get your av image from? Do you have a larger version I could look at?

The image in question is one I've had for years in my anime/comic pix archive. I'm pretty sure I got it from either the Venice anime ftp archive or wuarchive ftp archive back around '92 when I first got net access.

 

If not from an ftp archive, then I must have gotten it from a Lubbock, TX BBS when I was living there. I miss those BBSs a lot as they were a lot of fun. We used to have these really wild parties once a month...

 

Anyway, I have a few others, but that was the one I liked best. Here's the full sized image. =^.^=

 

 

But my favorite from that directory is the X-15 Cruise Basselope

 

 

I also have a pretty cool FLI/FLC of BTC around here somewhere with his head rotating on a cube. It's kind of hard to describe... =o.O=;;

 

I also made a really cool BTC prompt with flashing tongue for both DOS and Linux at the time. =^.^=;;

 

I still use the Linux one under Cygwin to this day. Sadly, the tongue doesn't flash in windowed mode as the Win32 console doesn't support flashing text unless you're in fullscreen and yet the cursor flashes. I always thought that was kind of stupid...

 

If anyone's interested in those let me know and I guess I can post it here. They're all pretty small.

 

And since this thread has gotten WAY OFF TOPIC... <giggle>=^.^=;;

Here's my old sig from back around then:

 

 

-----BEGIN GEEK CODE BLOCK-----

Version: 3.1

GCS d? s: a30 C+(+++) UL++++ P+>+++ L+++(++)>++++ E- W+ ____/| ACK!

N+(++) !o K+++ w+(---) O- M- V-- PS+ PE Y+ PGP++ t+ 5+ \'o.O' /

X+ R tv b++ DI+ D++ G e+>+++ h+(----) r++(*) z++ =(___)=

BobKat@jps.net - Hacking up hairballs near U....

------END GEEK CODE BLOCK------

<Kosh> Mihoshi is. </Kosh>

 

Ack!!! That just does NOT look right with the extra line spacing the forum uses. =o.O=;;

[Rybags]

Cheers.

 

You can use [ code ] [ /code ] without the spaces to force monospaced output. I tried it on that tag, but it didn't seem to make a lot of difference though.

[bobkat2769]

Cheers.

 

You can use [ code ] [ /code ] without the spaces to force monospaced output. I tried it on that tag, but it didn't seem to make a lot of difference though.

Funny you should have popped up just after I finished testing just that both ways. =^.^=;;

 

I thought it looked even worse with the code tag. I tried both ways (the nonbreaking spaces were a pain as the old &nbs; code didn't work) and used the better looking one.

 

Anyway, enjoy the pix!

 

BobKat

[bobkat2769]

Hi there,

 

I've got a piece of hardware that I'm trying to ID and could use some help.

 

I do know what it is, or is supposed to be. The case is labeled "RS232 Interface" and it has an SIO jack at one end and a DC power jack plus a DB-25 serial jack at the other end.

[clip!]

There's really not much to it. Two resistors, two diodes, two 100uF electrolytics, and two 14-pin ICs. The first one's a MC1489N DTL Quad Line Driver. The other I've no idea as the markings appear to have been sanded off. Maybe a PAL, no telling.

[clip!]

here in australia, we had a terminal program written by john nicholls called mega term. it had a simillar interface to what you have except it was built into the case of a 25 pin shell serial plug. The chips are a 74ls126 and the 1489. this interface also has one of the rs232 signals going to one of the interupt lines. Motor control is used to enable the gates on the 126. your interface could be similar and could use the rverter driver.

james

Got a schematic? That sounds a lot like what I have except for the much smaller case. That might even be what my unidentified chip is (74ls126). It's got the right number of pins to match it.

 

Thanks!

 

BobKat

[sup8pdct]

Hi there,

 

I've got a piece of hardware that I'm trying to ID and could use some help.

 

I do know what it is, or is supposed to be. The case is labeled "RS232 Interface" and it has an SIO jack at one end and a DC power jack plus a DB-25 serial jack at the other end.

[clip!]

There's really not much to it. Two resistors, two diodes, two 100uF electrolytics, and two 14-pin ICs. The first one's a MC1489N DTL Quad Line Driver. The other I've no idea as the markings appear to have been sanded off. Maybe a PAL, no telling.

[clip!]

here in australia, we had a terminal program written by john nicholls called mega term. it had a simillar interface to what you have except it was built into the case of a 25 pin shell serial plug. The chips are a 74ls126 and the 1489. this interface also has one of the rs232 signals going to one of the interupt lines. Motor control is used to enable the gates on the 126. your interface could be similar and could use the rverter driver.

james

Got a schematic? That sounds a lot like what I have except for the much smaller case. That might even be what my unidentified chip is (74ls126). It's got the right number of pins to match it.

 

Thanks!

 

BobKat

 

Here is the megaterm interface. the extra gate connected to command and proceed won't be on yours.

This is used by the megaterm software to detect the interface It may also not have the CD line detect.

 

James

[bobkat2769]

Hi there,

 

I've got a piece of hardware that I'm trying to ID and could use some help.

 

I do know what it is, or is supposed to be. The case is labeled "RS232 Interface" and it has an SIO jack at one end and a DC power jack plus a DB-25 serial jack at the other end.

[clip!]

There's really not much to it. Two resistors, two diodes, two 100uF electrolytics, and two 14-pin ICs. The first one's a MC1489N DTL Quad Line Driver. The other I've no idea as the markings appear to have been sanded off. Maybe a PAL, no telling.

[clip!]

here in australia, we had a terminal program written by john nicholls called mega term. it had a simillar interface to what you have except it was built into the case of a 25 pin shell serial plug. The chips are a 74ls126 and the 1489. this interface also has one of the rs232 signals going to one of the interupt lines. Motor control is used to enable the gates on the 126. your interface could be similar and could use the rverter driver.

james

Got a schematic? That sounds a lot like what I have except for the much smaller case. That might even be what my unidentified chip is (74ls126). It's got the right number of pins to match it.

Here is the megaterm interface. the extra gate connected to command and proceed won't be on yours.

This is used by the megaterm software to detect the interface It may also not have the CD line detect.

[Ding! Ding! Ding!] We finally have a winner! Well, sort of... =^.^=;;

 

Thanks again. Sorry to be so late to answer. I finally had some time today to take a closer look at both your schematic and the circuit I've got. I started doing up a schematic but gave up on it since it was taking way too much time and I started finding some really weird stuff that doesn't make much sense to me.

 

Like this earphone jack under the RS-232 port that connects both to the Vcc pin on the line buffer (Vcc could ONLY come to it from the jack) and to both control lines for one each of the line buffer and driver respectively. Lord knows what it hooked up to and what the circuit it was hooked up to was.

 

When I drew out what I found there and looked at it, I was like, "WTF????" and figured I've got much better things to with my time than try to puzzle out a circuit that probably didn't even work as it was wired by whoever built it originally. =o.O=;;

 

I certainly can't use it for anything at all because of that so at this point I think I should just scrap it for parts.

 

Other than that it's pretty much what you have in your schematic with a few 'interesting' differences that don't make a lot of sense to me. =o.O=;;

 

At least now I pretty much know what type of serial interface it was and if I might be able to put it to use for something else besides a modem. And it doesn't look like I can even use it for that. Anyway, that was pretty much all I wanted to know.

 

Thanks again,

 

BobKat

[sup8pdct]

That jack could be power for the unit. pics of both the top and bottom would be most helpfull.

 

James

[Fuji-Man]

Hi,

 

Just a guess, but maybe for a cash register drawer? I think those were run from serial ports at one time...

 

Fuji-Man

 

Hi there,

 

I'm sure it probably needs some kind of special driver. Anyone out there have any idea what it was supposed to hook up to and who made it?

 

Thanks much in advance,

 

BobKat =^.^=

[bobkat2769]

That jack could be power for the unit. pics of both the top and bottom would be most helpfull.

That was my first thought as well, but when I started looking at how it was wired that seemed very unlikely. Most of the circuit definitely takes all its power from the SIO port. I'm guessing it hooked up to some kind of circuit with a switch or a relay possibly built into the equipment it was hooked up to. Most likely a modem or dumb terminal of some kind.

 

Anyway, for the insanely curious.... =^.^= If any of you really want to mess with it and trace out the circuit I'd be more than happy to post hi-res pictures of the device inside & out. I'd kind of like to know for sure myself, but I'm not gonna spend a bunch of time on it.

 

So, let me know if you want me to post those pix and I'll try to get them up within 24 hours of hearing something.

 

later,

 

BobKat

[sup8pdct]

That jack could be power for the unit. pics of both the top and bottom would be most helpfull.

That was my first thought as well, but when I started looking at how it was wired that seemed very unlikely. Most of the circuit definitely takes all its power from the SIO port. I'm guessing it hooked up to some kind of circuit with a switch or a relay possibly built into the equipment it was hooked up to. Most likely a modem or dumb terminal of some kind.

 

Anyway, for the insanely curious.... =^.^= If any of you really want to mess with it and trace out the circuit I'd be more than happy to post hi-res pictures of the device inside & out. I'd kind of like to know for sure myself, but I'm not gonna spend a bunch of time on it.

 

So, let me know if you want me to post those pix and I'll try to get them up within 24 hours of hearing something.

 

later,

 

BobKat

 

Yes please, only if it isn't going to be too much trouble.

 

James

[bobkat2769]

That jack could be power for the unit. pics of both the top and bottom would be most helpfull.

That was my first thought as well, but when I started looking at how it was wired that seemed very unlikely. Most of the circuit definitely takes all its power from the SIO port. I'm guessing it hooked up to some kind of circuit with a switch or a relay possibly built into the equipment it was hooked up to. Most likely a modem or dumb terminal of some kind.

 

Anyway, for the insanely curious.... =^.^= If any of you really want to mess with it and trace out the circuit I'd be more than happy to post hi-res pictures of the device inside & out. I'd kind of like to know for sure myself, but I'm not gonna spend a bunch of time on it.

 

So, let me know if you want me to post those pix and I'll try to get them up within 24 hours of hearing something.

Yes please, only if it isn't going to be too much trouble.

No real trouble, except for the closeups. I had to retake those several times before they were usable. Those were a real pain as it would either overexpose the flash or just not want to focus that close.

 

Anyway, the camera had a lot of trouble with those and I sort of gave up on a good closeup of the serial port wiring. Only pins 2 & 3 are actually used and 7 is ground.

 

The two electrolytics are both 100uF 25VDC.

 

The two diodes, I can't make out the part number without removing them. I think they may be low value zeners as on both of them I got voltage drops of 0.7V forward biased and reverse voltage drops of 2.7V on one and 2.36V on the other. So they may be 2.5V zeners. Definitely silicon diodes of some sort.

 

The ICs are MC1489N (that's the marked package) and the unmarked package is probably a 74LS126. At least it looked like it was one as far as I had gotten tracing out the circuits myself. And those are the two used in the Megaterm interface that was referenced earlier. This circuit is very similar to that one.

 

If you do up a schematic I'd like to see it myself. Hopefully you can make heads or tails of it. Most of it makes sense to me, but not all of it. But then, when it comes to electronics I'm more of a repair than a design person. There's definitely some tricky klugery going on here that someone more designed oriented would understand that I don't. =^.^=;;

 

Okay, here are the pix of the device. =^.^=

 

[sup8pdct]

That jack could be power for the unit. pics of both the top and bottom would be most helpfull.

That was my first thought as well, but when I started looking at how it was wired that seemed very unlikely. Most of the circuit definitely takes all its power from the SIO port. I'm guessing it hooked up to some kind of circuit with a switch or a relay possibly built into the equipment it was hooked up to. Most likely a modem or dumb terminal of some kind.

 

Anyway, for the insanely curious.... =^.^= If any of you really want to mess with it and trace out the circuit I'd be more than happy to post hi-res pictures of the device inside & out. I'd kind of like to know for sure myself, but I'm not gonna spend a bunch of time on it.

 

So, let me know if you want me to post those pix and I'll try to get them up within 24 hours of hearing something.

Yes please, only if it isn't going to be too much trouble.

No real trouble, except for the closeups. I had to retake those several times before they were usable. Those were a real pain as it would either overexpose the flash or just not want to focus that close.

 

Anyway, the camera had a lot of trouble with those and I sort of gave up on a good closeup of the serial port wiring. Only pins 2 & 3 are actually used and 7 is ground.

 

The two electrolytics are both 100uF 25VDC.

 

The two diodes, I can't make out the part number without removing them. I think they may be low value zeners as on both of them I got voltage drops of 0.7V forward biased and reverse voltage drops of 2.7V on one and 2.36V on the other. So they may be 2.5V zeners. Definitely silicon diodes of some sort.

 

The ICs are MC1489N (that's the marked package) and the unmarked package is probably a 74LS126. At least it looked like it was one as far as I had gotten tracing out the circuits myself. And those are the two used in the Megaterm interface that was referenced earlier. This circuit is very similar to that one.

 

If you do up a schematic I'd like to see it myself. Hopefully you can make heads or tails of it. Most of it makes sense to me, but not all of it. But then, when it comes to electronics I'm more of a repair than a design person. There's definitely some tricky klugery going on here that someone more designed oriented would understand that I don't. =^.^=;;

 

Okay, here are the pix of the device. =^.^=

 

 

 

Have had a quick look and it appears the 2nd chip is a mc1488 rs232 driver. it requires both a positive and negative voltage of around 12V which would be supplied from the same place as the rs232 cable is plugged into. so it isn't anything like the rverter or megaterm interface.

 

James

[bobkat2769]

That jack could be power for the unit. pics of both the top and bottom would be most helpfull.

That was my first thought as well, but when I started looking at how it was wired that seemed very unlikely. Most of the circuit definitely takes all its power from the SIO port. I'm guessing it hooked up to some kind of circuit with a switch or a relay possibly built into the equipment it was hooked up to. Most likely a modem or dumb terminal of some kind.

Yes please, only if it isn't going to be too much trouble.

No real trouble, except for the closeups. I had to retake those several times before they were usable. Those were a real pain as it would either overexpose the flash or just not want to focus that close.

 

The two diodes, I can't make out the part number without removing them. I think they may be low value zeners as on both of them I got voltage drops of 0.7V forward biased and reverse voltage drops of 2.7V on one and 2.36V on the other. So they may be 2.5V zeners. Definitely silicon diodes of some sort.

 

The ICs are MC1489N (that's the marked package) and the unmarked package is probably a 74LS126. At least it looked like it was one as far as I had gotten tracing out the circuits myself. And those are the two used in the Megaterm interface that was referenced earlier. This circuit is very similar to that one.

 

If you do up a schematic I'd like to see it myself. Hopefully you can make heads or tails of it. Most of it makes sense to me, but not all of it. But then, when it comes to electronics I'm more of a repair than a design person. There's definitely some tricky klugery going on here that someone more designed oriented would understand that I don't. =^.^=;;

Have had a quick look and it appears the 2nd chip is a mc1488 rs232 driver. it requires both a positive and negative voltage of around 12V which would be supplied from the same place as the rs232 cable is plugged into. so it isn't anything like the rverter or megaterm interface.

Looking at the data sheet for the 1488 that actually makes some sense to me. Since even though the Atari SIO port could provide the Vcc+ from its +12V line it wouldn't be able to supply the Vcc- the 1488 needs. So an external power supply would required for the driver IC.

 

But what about those two diodes then? Like I said, they look to be 2.5V zeners by the voltage drops I got across them.

 

Interesting...

 

BobKat