SC131 Failed to communicate with terminal.

[John Matheson]

Once built my SC131 failed to communicate with my PC terminal program. All tests suggested the kit build was good and everything correctly soldered and in place. Sn experienced kit builder and fault finder I set about buzzing out the bus connections and other likely spots solder bridges may have occurred to find all OK. I then went for the Oscilloscope and it was not until I disconnected the RS232 interface from the board that the fault was obvious. I had substituted a TTL to RS232 module from the Arduino range of addon's which had worked fine with my Altaids 8085 SKU17494 computer kit but was excessively loading the port's TX and RX lines on the SC131 kit. I guess its the risk of buying the incomplete kit but also a wakeup call as most level rs232/TTL converters don't specify if they are low or hi impedance. A quickly made buffer of pairs of inverting gates gates on a 74HC14 got me going for the short term saving me from tossing the kit in the corner as a work in progress. Be warned its often the simple things that catch you out. 

[Steve Cousins]

Your conclusion is correct. It is likely this combination does not work due to the current limit (protection) resistors in SC131. These limit the current in and out of SC131 on the serial port lines. The resistors in SC131 are relatively high values for maximum protection and work fine with all the Serial to USB adapters I've tried. However,  if the connected device loads the signals enough it would result in a failure of SC131 to output characters. You could try replacing the 2k2 resistors with, say, 1k resistors. 

[John Matheson]

Thanks Steve for the suggestion re the 2.2K resisters. It was my initial thought to change or even remove them but harking  to my days with Motorola 6850 uarts I remembered the many with dead TX and RX ports so I decided to stay with a buffer. So far operation is 100% and I am enjoying using the kit.