VIA Peer Protocol

Wire protocol for a peripheral to exchange GPIO port and control-signal state with a VIA device (config type via/6522) over an attached transport.

Connection semantics

The VIA supports multiple concurrent peripheral connections over a multipoint transport (tcp: or unix:) — pty:/pipe: transports are point-to-point and can’t tag messages per connected client, so the config module rejects them for via/6522 at instantiation time. Just as on real hardware, it’s up to the peripherals themselves not to interfere with each other (e.g. two peripherals both driving the same output pin).

When a peripheral connects, the VIA immediately sends it a full state dump covering both ports and their control signals, so it starts with an accurate picture without having to wait for the next change. After that, whenever any connected peripheral changes VIA state — or a 6502 program does, by writing a port configured for output — every connected peripheral is informed of the change. Unrecognized peripheral input is silently ignored; there is no error signaling in either direction.

Message encoding — ASCII or Binary — is chosen per device at configuration time via the protocol attribute (protocol = "ascii" or protocol = "binary"; default ascii), not negotiated per connection. Every peripheral connected to a given VIA instance uses the same encoding.

[[devices]]
type = "via/6522"
address = 0x9000
protocol = "binary"
transport = "unix:/path/to/via.sock"

The ASCII encoding is useful for interactive sessions from a terminal or socket utility, for education or debugging, and for simple scripting. The binary encoding is compact and efficient, and is the better choice for a real peripheral implementation.

Message types

Six message types are defined:

1. Port State — sent by the VIA to convey the current state of Port A or B; may also be sent by a peripheral to configure the state of all pins of the subject port.
2. Ctrl State — sent by the VIA to convey the current state of the control pins for Port A or B; may also be sent by a peripheral to configure the state of both control pins for the subject port.
3. Reset Port — sent by a peripheral to reset any combination of bits in the specified port. The VIA sends this to convey bit-level state changes as needed. The message includes a mask byte identifying, with ones in the corresponding bit positions, which bits to reset.
4. Set Port — sent by a peripheral to set any combination of bits in the specified port. The VIA sends this to convey bit-level state changes as needed. The message includes a mask byte identifying, with ones in the corresponding bit positions, which bits to set.
5. Reset Ctrl — sent by a peripheral to reset either control pin (Cx1 or Cx2) for the specified port. The VIA sends this to convey changes in individual control signals. The ASCII message specifies the pin to reset; the binary message specifies a mask identifying which control bits to reset.
6. Set Ctrl — sent by a peripheral to set either control pin (Cx1 or Cx2) for the specified port. The VIA sends this to convey changes in individual control signals. The ASCII message specifies the pin to set; the binary message specifies a mask identifying which control bits to set.

ASCII protocol

Short strings of printable ASCII characters. A receiver must discard non-printable ASCII control characters (0x00–0x1F, 0x7F), spaces (0x20), and any byte with the high-order bit set, and must not distinguish upper case from lower case letters.

As an aid to human readability, the VIA separates distinct messages with a single space, and emits a canonical CR (0x0D) LF (0x0A) after every 72 characters of messages and spaces.

• p is a port identifier A or B
• u is a control pin identifier 1 or 2
• v is a bit state 0 or 1
• x is an ASCII hexadecimal digit 0..F

Binary protocol

Each message starts with a byte whose high-order bit is set. The upper four bits distinguish the message type; the lower four carry parameters. Port State, Reset Port, and Set Port messages are followed by one additional data byte.

• b is an arbitrary bit, 0 or 1
• p identifies the port; 0 for port A, 1 for port B
• u and v are the states of control pins Cp1 and Cp2 respectively; 0 or 1
• y and z are mask bits identifying whether control pins Cp1 and Cp2 respectively are affected by a reset or set operation; 0 or 1