Modem

This program automates telephone dialing by sending pulse sequences through the TS2068’s expansion port (I/O port 119), then monitors the same port for a connection signal before launching MTerm. Each digit is converted into a series of ON/OFF pulses via OUT 119 instructions, with the zero digit correctly mapped to 10 pulses at line 32. After dialing, the program polls IN 119 for the value 133 (line 165) to detect a carrier signal, plays a BEEP alert on connection, then patches a block of machine code in RAM via POKE statements before invoking it with PRINT USR 54016. The DATA block at line 300 contains 20 bytes of Z80 machine code patched into addresses 61193–61211, and several other POKEs at lines 220–260 modify MTERM.

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Program Analysis

Program Structure

The program is divided into four functional phases:

1. Input & dial: Lines 1–55 — prompt for a phone number string, then loop over each character to generate dial pulses.
2. Pulse generation subroutine: Lines 60–140 — a GO SUB routine that fires a pulse pairs through port 119.
3. Connection detection: Lines 160–180 — poll IN 119 up to 2000 times, branching on value 133 (carrier detected), otherwise resetting and redialing.
4. MTERM launch: Lines 200–305 — alert the user with BEEP, patch the resident terminal program in RAM, and invoke machine code via PRINT USR 54016.

Note that line 55 jumps to line 150, which does not exist; execution falls through to line 160. This is an intentional technique to enter the connection-polling loop.

Pulse Dialing via Port 119

Port 119 is the TS2068’s expansion bus control port (also used by the AY sound chip interface region and external hardware). Here it is used to toggle an attached relay or phone interface. Each digit produces a sequence of pulse pairs:

• OUT 119,e with e cycling 3→4 raises and lowers the line.
• PAUSE 2.5 (approximately 50 ms at 50 Hz) sets pulse width.
• OUT 119,1 and OUT 119,2 provide inter-pulse spacing (line 110–120).
• The digit zero is remapped to 10 pulses at line 32 (IF a=0 THEN LET a=10), matching the standard pulse-dial convention.
• PAUSE 35 at line 35 provides the inter-digit pause (~700 ms).

The initial OUT 119,31 at line 15 followed by PAUSE 50 appears to assert an off-hook condition before dialing begins.

Connection Detection

Lines 160–180 implement a polling loop. IN 119 is read up to 2000 iterations; if it returns the value 133, a carrier has been detected. If no carrier is found within the loop, OUT 119,0 hangs up, a pause is taken, and the program restarts at line 3 (re-displaying the dialing screen and redialing). The value 133 (binary 10000101) is hardware-specific to the attached modem or phone interface.

MTERM Patching and Machine-Code Launch

On successful connection, lines 220–280 directly patch a terminal program (MTERM) already loaded elsewhere in RAM. This is a sophisticated technique: rather than loading MTERM from tape at runtime, it is assumed to be pre-resident, and specific bytes are modified to configure it for the current session.

Lines	Address Range	Purpose
220	61157–61158	POKE 24, 6 — likely a JP offset or configuration byte
230	61170, 61172	POKE 184 — baud rate or port configuration bytes
240	61193–61211	19 bytes of Z80 machine code from the DATA block
250	61311–61331	Fill 21 bytes with 32 (ASCII space / Z80 NOP equivalent region)
260	61699	POKE 21 — likely a screen or protocol parameter

The DATA bytes at line 300 are: 254,147,27,95,254,147,27,95,234,246,50,193,154,89,193,247,105,105,121,195. Decoding these as Z80 instructions reveals patterns such as CP 147 (FE 93), DEC DE (1B), LD E,A (5F), JP Z,nnnn (CA/C2), and LD A,C (79), suggesting a receive/transmit character dispatch routine. The terminal is then invoked with PRINT USR 54016, calling machine code at address 54016.

Notable BASIC Idioms and Anomalies

• LET a=VAL a$(i) extracts a single digit from the phone number string by taking a one-character substring and converting it — a common Sinclair BASIC idiom since direct character-to-digit conversion is otherwise unavailable without CODE.
• The connection-alert loop at lines 200–218 uses INKEY$ polled without PAUSE 0; it loops via GO TO 200, replaying the BEEP sequence each pass, which gives a repeated beep until Enter is pressed.
• LIST at line 305 after launching the machine code is unreachable under normal execution and appears to be a debugging remnant.
• The FLASH 1 attribute in both the dialing message (line 19) and the connected message (line 165) provides visual feedback without any additional logic.
• PAUSE 2.5 at line 90 uses a non-integer argument; the system truncates or rounds this to the nearest frame count (2 or 3 frames, ~40–60 ms), giving approximate pulse timing for the phone line.