> I couldn't spot how they did level shifting to RS232 levels. The SI > signal appears to go to a 7400 next to one of the Uarts.
There are a couple of 8 pin ICs near the 10 pin headers, they look to be 1458 dual op-amps (I can't exactly make out the numbers...). Op-amps were often used as RS232 drivers. Give them +/-12V supplies, feed a TTL signal into one of the inputs, connect the other input to a voltage between the TTL levels (potential divider from the 5V line) and apply no negative feedback. The Op-amp output will swing between the supply rails. -tony
