Vs. System

The Vs. Unisystem and Vs. Dualsystem are arcade system boards based on the NES.

Palette

There are several different RGB PPUs used in Vs. games. To determine which PPU is used, read the PPU type byte of the NES 2.0 header if available; otherwise, use the hash of the PRG and CHR ROM data.

2C03

This PPU is used in Duck Hunt and Tennis, as well as the PlayChoice, Famicom Titler, and Famicom TVs. Its colors closely resemble those of the 2C02 in the standard NTSC NES.

2C04

There are four versions of this PPU with different palettes. This was used as a form of copy protection, so that games would have wrong colors if someone were to burn a new PRG ROM and CHR ROM and put them on the game PCB.

2C05

This PPU has the same colors as the 2C03, but it swaps the meanings of $2000 and $2001 and returns a constant identifying value in bits 4-0 of $2002. This is also for copy protection.

Registers

Registers $4016 and $4017 have additional bits related to coin insertion and difficulty switches, and $4020 is a new register. The controllers are swapped: $4017 on the left and $4016 on the right. The 2C05 swaps PPUCTRL and PPUMASK. Otherwise, all registers have the same meanings as on the NES or PlayChoice.

Controller and CHR ROM bank ($4016 write)

7  bit  0
---- ----
xxxx xCRS
      |||
      ||+- 1 then 0: Request a report from the joysticks or Zapper
      |+-- In the DualSystem, connects to 1- /IRQ of the other CPU and 2- when high on the primary
      |      specifies that 2KiB of RAM is mapped from $6000-$7FFF, else mapped for the secondary CPU
      +--- Select 8 KiB CHR ROM bank for PPU $0000-$1FFF (mapper 99 games only)
           Note: In case of games with 40KiB PRG-ROM (as found in VS Gumshoe),
                 the above bit additionally changes 8KiB PRG-ROM at $8000-$9FFF.

Note that VS Super Mario Bros. relies on having the shared memory granted to it. The bundled RP2A04 is nothing but a jumper that ties the primary socket's OUT1 pin to ground.

Controller 2 data, coins, and DIP switches ($4016 read)

7  bit  0
---- ----
xCCD DSxB
 ||| || |
 ||| || +- Buttons for player 2 (A, B, 1, 3, Up, Down, Left, Right)
 ||| |+--- Service button (commonly inserts a credit)
 ||+-+---- DIP switches "2" and "1", respectively
 ++------- Coin inserted (read below)

The "coin inserted" signals are the contact switches in the coin acceptors. The default arcade machine wiring connects both coin acceptors in parallel, and only to the $20s bit, so that bit is the only one of the two that goes high when a coin is inserted in either acceptor. However, games MUST check both bits because the coin acceptors could have been connected independently. Some games refuse to grant a credit if the "coin inserted" signal is true for too long, to protect against malicious arcade-goers.

The coin collectors will read as true for approximately 1/20th to 1/15th of a second. A game should check the registers at least every other NMI to be assured of not eating the player's coins.

The service button is debounced with an RC network: after the button is released, it should take somewhere between 3 to 6 milliseconds until it reads as false. After the capacitor has been fully charged, it should take between 1 and 1.4 milliseconds until it reads as true. The game need not poll the service button with any frequency, since the button can be held by the arcade operator for an arbitrary amount of time.

Games may, and even probably should, parse the two "coin inserted" bits independently: emulators should NOT set both bits high when a single virtual coin has been inserted. For example, VS Super Mario Bros. gives 2 credits if both coin bits are set.

Unlike previous advice, it is not necessary to acknowledge a coin insert by driving the coin counter, but the arcade operator would be cross if the game didn't!

Controller 1 data, DIP switches, and watchdog timer ($4017 read)

7  bit  0
---- ----
DDDD DDxB
|||| || |
|||| || +- Buttons for player 1 (A, B, 2, 4, Up, Down, Left, Right)
++++-++--- More DIP switches (7 down to 2)

The game must read from $4017 at least every 1.2 seconds, or the CPU, PPU, and bit at $4020 are reset.

Coin counter ($4020-$403F, &c)

The game is expected to keep track of the total number of coins inserted by toggling this line every time a coin is seen while reading $4016.

15   address 4    0  7  bit  0
---- ---- ---- ----  ---- ----
010x xxxx xx1x xxxA  xxxx xxxC
                  |          |
                  |          +- (write) 1, delay, 0: Increment coin counter
                  +------------ (read) Same as write, but see below

The port is mirrored across the entire range of $4020-$5FFF and may interfere with mappers that put ports, ROM, or RAM in this range. When the latched value is 1, it drives an electromagnet in the coin counter. Writing 1 then 0 will increment the counter. The electromagnetic counter can vary from machine to machine, but the least common denominator is a 10Hz DC coin counter: driving the signal high for 50ms (3 vblanks) and then low for 50ms is guaranteed to work. Some counters may be able to be driven faster: one requires as little as 16ms high and 22ms low. This register is explicitly cleared on powerup and reset.

Reading from the register effectively writes the value of open bus (usually the LSB of the address, but other values could appear from e.g. DMC interrupts) No games use any of this bit's mirrors, or try to read from it.

See also

• VS System mainboard schematic
• VS UniSystem cabinet harness hookup diagram
• VS UniSystem conversion kit installation guide for converting an older single-game Nintendo arcade machine to a VS UniSystem
• Lidnariq's Vs. System's characterization test ROM and Memblers's test results for it.

The hardware used by games released during the Vs. System's commercial life is equivalent to the following mappers:

• iNES Mapper 099 (games provided without a daughterboard)
• iNES Mapper 001 (MMC1)
• iNES Mapper 002 (UxROM)
• iNES Mapper 067 (SUNSOFT-3)
• iNES Mapper 075 (VRC1, erroneously iNES Mapper 151)
• iNES Mapper 206 (Namco 108, erroneously MMC3/iNES Mapper 004)