Olivetti Prodest PC1: CRT.COM vs PERITEL.COM, Boot Video Mode, and the Hidden 160×200×16 Mode Explained
The Olivetti Prodest PC1 has a unique dual‑video system that differs significantly from standard IBM CGA. Because of this, Olivetti supplied two utilities — **CRT.COM** and **PERITEL.COM** — to switch between its two timing systems. Understanding how these interact with the BIOS default mode also explains why some monitors flicker, lose sync, or show “hidden” graphics modes.
1. BIOS Default Video Mode (before running any tools)
When the PC1 boots, the BIOS initializes the video hardware in a **safe, generic CGA‑compatible mode**:
- 80×25 text
- Standard IBM‑style CGA timing
- Output on the RGBI monitor port (DE‑9)
- No SCART/TV timing
- No Olivetti‑specific register tweaks
This mode is extremely LCD‑friendly, which is why most LCD monitors show a stable picture immediately after power‑on.
**Important:**
The BIOS default mode is *not* the same as the mode that CRT.COM sets.
2. CRT.COM — Olivetti’s Custom Monitor Mode
`CRT.COM` switches the PC1 into **Olivetti’s own CGA monitor profile**, which differs from the BIOS default.
It reprograms the CRTC registers with Olivetti‑specific values:
- different horizontal/vertical totals
- different sync widths
- different character clock
- different palette initialization
- 80‑column text mode
- SCART logic disabled
Even though it is still “monitor mode,” the timing changes are enough to make many LCDs **blink or resync** when CRT.COM is executed.
On a real CGA RGBI CRT monitor (IBM 5153 or compatible):
✔️ Works perfectly
✔️ No flicker
✔️ No sync issues
On an LCD:
⚠️ Brief black screen or resync
⚠️ Some LCDs reject the mode entirely
3. PERITEL.COM — TV/SCART Mode
`PERITEL.COM` switches the PC1 into **TV timing** for SCART output:
- 15.6 kHz horizontal
- 50 Hz vertical
- 40‑column text mode
- SCART RGB blanking (pin 16)
- Composite sync (pin 20)
This mode is intended for televisions, not computer monitors.
Most LCDs cannot sync to it at all.
4. The Hidden 160×200×16 Graphics Mode
The PC1 includes a non‑standard graphics mode:
160×200 in 16 colors
This mode is not part of IBM CGA, but it is implemented in the PC1’s custom video hardware.
Key facts:
- The mode exists in all three timing systems (BIOS default, CRT.COM, PERITEL.COM).
- It is not tied to SCART — it’s simply easier to see on a TV.
- In monitor mode (CRT.COM), many LCDs cannot sync to the timing used by this mode.
- On a real CGA CRT monitor, the mode works normally.
- On a TV via PERITEL, the mode is usually stable and visible.
Monitor compatibility:
The attachment PC1 Video modes.png is no longer available
Why?
- CGA CRTs accept TTL RGBI and have wide sync tolerance.
- EGA monitors expect analog RGB and different sync frequencies.
- LCDs expect strict VGA‑style timing and often reject custom CGA modes.
Final Notes
- The PC1’s video hardware is almost CGA‑compatible, but with enough differences to break assumptions made by some games and LCD monitors.
- The hidden 160×200×16 mode is a genuine hardware feature, not an artifact of SCART mode.
- For the most accurate experience, a true CGA RGBI CRT monitor is the correct display device for the PC1’s monitor mode.
About NTSC Artifacting and Composite Output on the Olivetti PC1
The Olivetti Prodest PC1 does not support NTSC composite color, and therefore it cannot produce NTSC artifact colors used by many CGA games.
Why:
The “composite” pin on the PC1’s DE‑9 connector is monochrome only (luminance).
It has no colorburst, no chroma, and no NTSC/PAL encoding.
It behaves like the monochrome composite output on Hercules/MDA cards.
Even in SCART/TV mode:
The PC1 outputs RGB, not composite video.
The scan rate matches PAL (15.6 kHz / 50 Hz), but the signal is still pure RGB, not PAL composite.
Result:
Games that rely on NTSC artifacting (Ultima, Digger, King’s Quest I, etc.) will only show black‑and‑white or striped graphics on the PC1, because the hardware simply does not generate NTSC composite color.
