Got to the bottom of the serial port issue with SBC6120, added air vents to 1130 Expander box

 AIR VENTS DRILLED INTO THE 1130 EXPANDER BOX CASE

I added side holes to draw in fresh air and top holes above the power supply fan which will exhaust the hot air from the case. Nothing else to do until the tool for extracting and reinserting the Amphenol M series sockets arrives. 

WHIPPED UP TTL TO RS232 LEVEL SHIFTERS FROM SPARE PARTS

Since the SBC6120 makes use of the voltage levels of RS232, the line can swing as high as +12V and as low as -12V, a definite problem when attached to a TTL part that expects voltages in the range of 0 to +5. I had some old chips in my cabinets that I put on a breadboard such that I could convert the transmit output from RS232 and take a TTL receive line and drive it to RS232 levels. 

I used a SN75150 dual driver chip and an MC1489 quad receiver chip to handle the conversions. The first takes +12 and -12 supply rails, the latter uses 5V. I jumpered the SBC6120 lines to the breadboard and then jumpered the TTL side to a USB to TTL adapter I owned. I used a voltmeter and the oscilloscope to confirm the proper levels and encoding of characters.

RS-232 and TTL conversion chips wired to rails

Still, however, I saw nothing but gibberish on the Putty terminal program. I checked with the Arduino serial monitor and the output was the same. Commands keyed in didn't see to be decoded by the SBC6120 nor responded to.

Just to throw in some amusement, one of the ground connections came loose which lead me on a ten minute wild goose chase debugging things until I realized that the RS232 to TTL receiver chip wasn't working because it had nothing on the 5V rail. 

As I looked more carefully at the scope image, the timing of the bit cells looked off so I did some calculations. Aha! The SBC6120 was jumpered for 300 baud but was operating at roughly 110. That triggered a distant memory of my having modified the SBC with a different timing crystal to achieve compatibility with old teletype and typewriter equipment. 

A quick look at the oscillator chip confirmed the issue. It was operating at 1.8432 MHz while the original from the schematic was a 4.9152 MHz oscillator. That gives a ratio of .375 for the serial stream, converting the 300 baud of the design down to 112 baud. 

I changed the oscillator to create 110 baud support

I happened to look in my parts bins and found a bag with an oscillator marked "original 4.9152 crystal". How convenient. I swapped the parts out, configured the jumper for 9600 baud operation, and got clean output on Putty set to that speed. 

I found the old oscillator part!

That is more like it!

IOB6120 HANGING THE SBC6120 STARTUP

Unfortunately, when I add the IOB6120 board, the SBC6120 system hangs in the memory test at startup. I was unable to get it past that point. When I hit the ESC key at startup, the monitor didn't start so that I had no way to enter the FL command to load the flash contents. 

At this point, having checked the soldering very carefully and attempted to move every pad with a dental pick, I will have to move into debugging to figure out what is going wrong. Keep in mind that five of the chips on the board were procured through eBay and thus have a reasonable risk they are counterfeit, which could result in a hang like this if they tie down bus lines or inject spurious outputs. That is, the Flash chip and four SRAM chips are suspect.