Introduction⌗

This a clone of the original Titan QX-PC by Titan Technologies Inc. After managing to track down and purchase an original I decided it would be interesting to reverse engineer the board to allow others to more easily (and cheaply) get a QX-PC of their own.

I also switched from using DRAM to a single 512k SRAM module. This simplified the design and meant you no longer needed to stack one to two memory boards on the base card, each 256k, in order to supply the memory.

Another change was to swap out the Bipolar PROM that was being used for IO decoding with a GAL16V8, this means with the exception of the clock generator IC and the 8088 itself, all the parts for this are easily purchased via digikey or mouser.

Schematic⌗

With the QX-PC board all access to the hardware is controlled by the Z80 processor. Whenever the 8088 performs an IO request it generates an interrupt to the Z80, IO Port 0x400 is special and allows the 8088 BIOS to pass messages between the 8088 and Z80 processors. Normal memory access between addresses 0xB0000 and 0xBFFFF also generate an interrupt and allow the Z80 to handle the memory mapped graphics used ny DOS.

Whenever an interrupt is generated the READY line of the 8088 is deasserted to allow the Z80 to handle the interrupt before the 8088 resumes processing. A Z80 interrupt is cleared by writing a value into the IO port 0xD1 from the Z80 interrupt handler. This value gets returned to the 8088 when it was doing an IO or memory read.

Bill of Materials
Ref Description Part
C1-C20 0.1uf Capacitor RDER72A104K1S1H03A
C21,C22 30pf Capacitor C315C300F3G5TA
R1-R3 220 Resistor CFR-25JR-52-220R
RN1 4.7k Resistor Network 4608X-AP1-472LF
U1 74LS245 SN74LS245N
U2 512k SRAM AS6C4008-55PCN
U3 74LS75 SN74LS75N
U6 28C64 AT28C64B-15PU
U7 74LS04 SN74LS04N
U8 74F08 SN74F08N
U9 74LS21 SN74LS21N
U10,U11 74LS74 SN74LS74AN
U4,U5,U13,U14,U16,U17 74LS373 SN74LS373N
U18 Clock Generator CP82C84
U15,U19 GAL16V8 ATF16V8B-15PU
U20 CPU 8088
Y1 15 Mhz Crystal HC-49/U-S15000000ABJB

CP/M Support⌗

The CP/M support for this comes in the form of the QX-PC app which is run from CP/M and will allow booting up an MS-DOS disk and manages communications between the Z80 processor and the 8088.

Since I have only been able to track down a disk image containing the software for the version 3 board I have had to make a slight modification to it in order to allow it to work with the version 2 board this clone is based on.

I have included an disk image containing the modified executable in my github repository.