8556 / 8557 SX Planar
U22-U25 Toshiba TC511665Z-80H 40 pin ZIP VRAM Functionality
J7 has a little sticker over it that says "J18"
The 57SX supports up to 16MB of 70-ns memory (all DMA addressable) on the planar in three SIMM sockets. The memory controller automatically supports enhanced performance via memory interleaving with all 2MB or all 4MB SIMMs. One, two or three 2MB SIMMs provide memory interleaving. Two or three 4MB SIMMs also provide memory interleaving. The 8MB SIMMs do not provide interleaving.
Video is provided via the enhanced 16-bit industry-standard VGA.
Supports 720KB, 1.44MB and 2.88MB diskettes
8557SX Bus Adapter
Dotted area on J1 Solder pads for a BVE segment
If you have a fantasy about swapping a Lacuna riser and planar into your 57SX, forget it. The Lacuna riser has two mounting slots where the screws are both horizontal, while the 57 has a vertical screw and a horizontal screw. The lacuna mounting holes at the front don't match.
> This one has actual sockets installed though, not empty pads...?
Some of the later Greenock variants of the 8556/8557 planar even lack these sockets and have only soldering spots. That was what my comment referred on.
You might however try to install some chips there for 512K and see if you can enable the mode 2Eh - that's 640 x 480 / 256 colors from a 256K palette at 60Hz vertical / 31.5 KHz horizontal. The system should choke out at Mode 12h (640 x 480 / 16 out of 256K, 60Hz / 31.5 KHz) and 13h respectively (320 x 200 / 256 out of 256K, 70Hz / 31.5KHz).