This is documentation for the ZMODEM implementation in Tera Term. This documentation was created to help understand ZMODEM and does not cover all ZMODEM specifications.

The following items were referenced.

• The ZMODEM Inter Application File Transfer Protocol / Chuck Forsberg
• lrzsz
  • defact standard impliment
  • lrzsz/doc/zmodem.doc

General transmit sequence

sequenceDiagram
participant s as Sender
participant r as Receiver
s->>r: ZRQINIT
r-->>s: ZRINIT
opt option
s->>r: ZSINIT
r-->>s: ZACK
end
loop files
s->>r: ZFILE
alt next file
r->>s: ZSKIP
else
r->>s: ZCRC (ZMCRC option)
s-->>r: ?
end
loop one file
r->>s: ZRPOS
s-->>r: ZDATA
end
s-->>r: ZEOF
r->>s: ZRINIT (receive file is ok)
end
s->>r: ZFIN
r->>s: ZFIN
s->>r: "OO" (0x4f 0x4f)

The following frame is not implemented in Tera Term.

• ZCRC 0x0d
• ZCHALLENGE 0x0e
• ZFREECNT 0x11
• ZCOMMAND 0x12
• ZSTDERR 0x13

3 types

• Header
  • Header only frame, Data Subpacket length = 0
• Header + Data Subpackets
• Header + Data Subpackets + Footer
  • Hex Header only

• The beginning of frame is the header
• There are two types of headers + HEX
  • 16 Bit CRC Binary Header
  • 32 Bit CRC Binary Header (Tera Term:send not impliment)
  • HEX Header (Based on 16bit CRC binary header)
• Use either HEX header or Binary header depending on the situation or frame type.
  • Considering communication channels that are not 8-bit clean, ZRQINIT and ZRINIT must use Hex Header.

10 byte length

• 16bit CRC = CRC-1 * 2^8 + CRC-2
• ZF? flag bytes
• ZP? position bytes

12 byte length

• 32bit CRC = CRC-1 * 2^(83) + CRC-2 * 2^(82) + CRC-3 * 2^8 + CRC-4

• Hex Header is usable on communication channels that are not 8-bit clean.
• Based on 16bit CRC binary header
• When receiving Hex Header, ignore MSB(0x80)

• convert binary -> "01234567890abcdef" (upper case hex are not allowed)

footer

• Datas(CR,LR,etc) following data subpacket are discarded until the next frame (starting with '*'(ZPAD 0x2a)).

• add CR/LF (one or two characters)

  • LF(0x0A)
  • CR(0x0D) LF(0x0A)
  • CR(0x0D) LF|0x80(0x8A)
    • 0x0D 0x8A is defact standard
    • MSB is ignored
  • CR/LF aids debugging from printouts

• add XON(0x11)

  • expect frame type is ZFIN or ZACK.
  • XON for releases from spurious XOFF flow control characters

• length is 0...1024 bytes
• convert ZDLE(0x18)
  • ZDLE(0x18) -> ZDLE(0x18) ZDLEE(0x58)

• ASCII Encoded Data Subpacket is used in 7 bit environment.
• ASCII Encoded Data Subpacket follows Hex Header.

To avoid sending control codes in communication channel, convert the control codes.

send

• When sending following data
  • ZDLE(0x18)
  • DLE(0x10) or 0x90(DLE|0x80)
  • XON(0x11) or 0x91(XON|0x80)
  • XOFF(0x13) or 0x93(XOFF|0x80)
  • '@'(0x40) or 0xc0('@'|0x80)
  • CR(0x0d) or 0x8d(CR|0x80)
  • ((data & 0x60) == 0) && (ESCCTL == 1)
    • ESCCTL is in ZSINIT frame
• send ZDLE and data inverted bit 6
  • send ZDLE(0x18) and (data ^ 0x40)
  • ZDLEE(0x58) = ZDLE ^ 0x40
    • ZDLE -> ZDEL ZDLEE

receive

• ignore
  • XON(0x11) or 0x91(XON|0x80)
  • XOFF(0x13) or 0x93(XOFF|0x80)
• receive ZDEL data
  • data ^ 0x40

• Frame descriptions are written in 32 Bit CRC Binary Header.
• In some cases, frames may use a HEX header.

• HEX Header
• Sender→Receiver

flags (ZF0...ZF3)

• HEX Header
• Receiver→Sender

flags (ZF0...ZF3)

MaxDataLen = (ZP1 << 8) + ZP0

Tera Term

• ESCCTL=1 when TERATERM.INI ZmodemEscCtl=ON

• (Binary header)
• Sender→Receiver

flags (ZF0...ZF3)

• (Binary header)
• Ack to ZRQINIT ZRINIT or ZSINIT

• Binary/Hex Header
• Sender→Receiver

• MODIFICATION DATE and FILE MODE is option

flags (ZF0...ZF3)

• HEX header
• Receiver→Sender

• Binary header
• Sender→Receiver

• Binary header
• Sender→Receiver

• Binary header