Reference data for the Epson CX-21 acoustic coupler, gathered from my own unit. Values marked “measured” come from that one unit. For the full story of how I traced and tested the board, see my reverse-engineering notes.
Specifications
| Item | Value |
|---|---|
| Type | Acoustic coupler (modem using a telephone handset) |
| Introduced | About 1983–84 |
| Standard | ITU V.21, 300 baud, full/half duplex |
| Interface | RS-232, DB25 Female (DCE) |
| Modes | Originate / Answer; Half / Full duplex; Test position (loopback) |
| Indicators | Power LED (blinks on low battery); Ready LED (carrier detected) |
| Power | 4-cell NiCd pack (marked RB-105, ~4.8 V) or DC adapter via jack. The jack should be center-negative as all Epson adapters of the era, modern adapters (center-positive) will not work. |
| Current draw | ~140 mA at 4.8 V idle, ~0.67 W (measured) |
| Board | “ACM-21 BOARD” |
| Works with | Epson portables such as the PX-8 and HX-20 |
Power
| Item | Value |
|---|---|
| Battery node range | 3–5.6 V; +5 V rail stays regulated from 3.0 to 5.2 V |
| Logic supply | +5 V from a transformer-based DC/DC converter (4.98 V measured) |
| RS-232 / op-amp rails | +8.4 V / −7.7 V (+8.36 V / −7.68 V measured) |
| Analog negative rail | −5 V from a 79L05 |
| Charging | Adapter → 8.2 Ω 2 W resistor (R1) → diode (D13) → battery. About 70 mA net into the pack with the Epson adapter (roughly C/10). |
| Epson adapter | ~9 V open circuit, ~6–7 V at 600 mA (measured) |
| Low battery | Power LED blinks below about 4.7–4.8 V. Latched: clears only after switching off for about a minute. |
V.21 frequencies
| Channel | Mark (1) | Space (0) |
|---|---|---|
| Originate (ch. 1) | 980 Hz | 1180 Hz |
| Answer (ch. 2) | 1650 Hz | 1850 Hz |
Not compatible with Bell 103 modems, which use different frequencies and reverse the mark/space order.
Main components
| Part | Function |
|---|---|
| Epson E03010A | Custom CMOS control chip (switches, RS-232 handshaking, LEDs, low-battery latch) |
| Epson E03020A | Custom modem chip; appears to handle FSK generation, demodulation and carrier detect |
| Toshiba TC4049BP | LED driver and RS-232 line receiver |
| TI SN75188N | RS-232 line driver |
| 3 × NEC µPC1458C | Dual op-amps: filters, microphone preamp, speaker driver |
| Mitsubishi M51204 | Voltage comparator; likely the battery detector or the converter controller (unconfirmed) |
Connecting to a computer
The Epson PX-8 user manual covers acoustic couplers in section 4.10 and lists cable #724 for couplers with a DB25 connector.
History
A related model, the CX-20, also exists; the Computer History Museum holds its operations manual (catalogue 102638123).
Documentation
- Original CX-21 manual: held in the Centre for Computing History archive (catalogue CH13135), not available online.
- No schematic is known to be available. If you have one, or the manual, please get in touch.