RS-232 OUT from 88-2SIOR1 Port 1 Address 18d/19d

[Walt Perko]

Hi, 

Trying to write a new program, I did some testing from one of the two computers involved with the program.  

The Altair 8800c with an 88-2SIOR1 on Port 1 Address 18d/19d I can send data via my DUMBTERM.BAS to the other computer a ZAltair Z80c also with an 88-2SIOR1 on Port 1 Address 18d/19d 

But when I try to send DATA from the ZAltair Z80c back to the Altair 8800c there is no data flow.  

The cable is a straight through RS-232 DE9 Male to DE9 Female to a Null Modem Adaper to the other computer which is a DE9 to DE9 Male into the 88-2SIOR1 on Port 1 Address 18d/19d 

Checking with my oscilloscope, I see data from the Altair 8800c on PIN 2

on the ZAltair Z80c from the Null Modem adapter PIN 3 I see data ... It seems the DATA should be going in that direction, but nothing shows up on the console.  

It's the exact same setup on both computers using the exact same  DUMBTERM.BAS program.  Yet data moving is only one direction ???  

I also tried a Null Modem cable DE9 to DE9 ... 

.

[Patrick Linstruth]

Why are you bothering with a null modem adapter?

The 2SIOR1 was designed to generally eliminate the need to use them.

You need one IDC to DE9-F attached to the DCE port on one 2SIOR1 and one IDC to DE9-M attached to the DTE port on the other 2SIOR1.

Then all you need is a straight DE9-M to DE9-F cable. No null modem adapters required.

If you are going to deviate from that, then you’re not using the boards as designed and will need to provide more detail of exactly how each cable and adapter is wired.

Set the baud rate on both boards to 110 and when you type a character, the TX light should blink on the sending computer and the RX light should blink on the receiving computer. At 110, it will be obvious. If the TX light doesn’t blink on the transmitting computer, you have a software problem. If it doesn’t blink on the receiving computer, you have a cable problem. If it does blink, and you still don’t get a character, either DCD isn’t active on the receiving 2SIOR1 or you have a software problem.

On Jun 4, 2026, at 8:39 PM, Walt Perko <r4r...@gmail.com> wrote:

Hi, 

Trying to write a new program, I did some testing from one of the two computers involved with the program.  

The Altair 8800c with an 88-2SIOR1 on Port 1 Address 18d/19d I can send data via my DUMBTERM.BAS to the other computer a ZAltair Z80c also with an 88-2SIOR1 on Port 1 Address 18d/19d 

But when I try to send DATA from the ZAltair Z80c back to the Altair 8800c there is no data flow.  

The cable is a straight through RS-232 DE9 Male to DE9 Female to a Null Modem Adaper to the other computer which is a DE9 to DE9 Male into the 88-2SIOR1 on Port 1 Address 18d/19d 

Checking with my oscilloscope, I see data from the Altair 8800c on PIN 2

on the ZAltair Z80c from the Null Modem adapter PIN 3 I see data ... It seems the DATA should be going in that direction, but nothing shows up on the console.  

It's the exact same setup on both computers using the exact same  DUMBTERM.BAS program.  Yet data moving is only one direction ???  

I also tried a Null Modem cable DE9 to DE9 ... 

<20260604-D9 Adapting.jpg>

<DE9 Male-Female Null Modem Adapter.jpg>

<IMG_9881-D9 Null Modem Adapter-c2K.jpg>

.

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<20260604-D9 Adapting.jpg><DE9 Male-Female Null Modem Adapter.jpg><IMG_9881-D9 Null Modem Adapter-c2K.jpg>

[Walt Perko]

Hi, 

I didn't run cables from the 88-2SIOR1 DTE ports on either computer, so I'm just going DCE to DCE.  

The problem is the DATA only flows one direction.  

I do have some FC10 to DE9 cables on order from AliExpress ... both Male DE9's and Female DE9's ... They should be here in a week or so.  

.

[Walt Perko]

Hi, 

I don't understand why if the cables are straight through RS-232 DE9 Male to DE9 Female with a Null Modem Adapter in between to the other computer which is also a DE9 to DE9 Male into the 88-2SIOR1 on Port 1 Address 18d/19d doesn't work bi-directional.  

Altair 8800c DCE to Null Modem to DCE ZAltair Z80c functions.  

ZAltair Z80c DCE to Null Modem to DCE  to Altair 8800c FAILS!  

.

[Patrick Linstruth]

They fail because either your cabling is wrong or there is something wrong with your program.

Did you look at the LEDs on the 2SIO boards like I suggested? Those LEDs were put there for you.

 

What are your /DCD and /CTS switches set to on both boards?

Have both ports on both boards worked correctly in the past?

Maybe post your BASIC program too.

Sent from my iPhone

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