On 11/3/2013 1:25 PM, Win Heagy wrote: The 9901/9902 combo is a bit of a giveaway. Find a copy of the 9900 Family Systems Design Guide for more info e.g. at https://archive.org/details/9900MicroprocessorSeriesFamilySystemsDesignData… There, you will learn that the TI 9900 family (whose processor chips included the 9900, 9989, 9995 (?), and others) was a bit of an oddity in various ways (though that particular manual was first class). One oddity was its IO mechanism, using a single-bit-wide serial IO bus called the CRU. IO chips designed for the CRU bus (including the 9901 "system controller", and the 9902 UART, and many others, were realistically only usable with other 9900-family chips. The 9900 architecture was used in the 990 family of "minicomputers" including the 990/4 and its bigger brother the 990/10. TXDS was the "OS" (being generous again here) on the /4; something called AMPL ran on the /10. There are some pictures around the net, and a few writeups. I can't tell from the one picture (where is the other?) if that's what you've got, and I can't remember whether the /4 we had at work used the 9901 and/or 9902 on its serial cards. There is a 990/4 Maintenance Guide (and other 990-family stuff) on bitsavers which might be worth a look, at or near http://bitsavers.trailing-edge.com/pdf/ti/990/990-4/ The TI99/4 and related home computer boxes are very different physically although likely use chips from the same family. Apparently there was a range of external peripherals. You'll work it out. Have a lot of fun, John