25 pin D-SUB male connector at the DTE (Computer).
25 pin D-SUB female connector at the DCE (Modem).
RS-366 (also published as EIA-366) is an automatic calling unit (ACU) interface standard that defines how a Data Terminal Equipment (DTE) controls an automatic calling modem — referred to as an Automatic Calling Equipment (ACE) — to dial telephone numbers without human intervention. The standard uses RS-232 electrical signalling over a 25-pin DB-25 connector, and transmits each dialled digit as a 4-bit Binary Coded Decimal (BCD) value in parallel across four dedicated lines. The international companion standard published by the CCITT was Recommendation V.25. RS-366 was largely superseded by the Hayes AT-command modem interface, which embedded auto-dialling control within the data channel itself.
| Pin | Function | Description | Circuit EIA |
|---|---|---|---|
| 1 | unused | ||
| 2 | Digit Present | A signal given to the ACE indicating that the digit lines contain a digit | DPR |
| 3 | Abandon Call and Retry | An indicator signal from the ACE that it could not make a connection. Could be “busy”. | ACR |
| 4 | Call Request | A signal from the DTE that tells the ACE to go “off hook” | CRQ |
| 5 | Present Next Digit | A signal from the ACE to the DTE to indicate that the ACE is ready to receive the next digit. | PND |
| 6 | unused | ||
| 7 | unused | ||
| 8 | unused | ||
| 9 | unused | ||
| 10 | unused | ||
| 11 | unused | ||
| 12 | unused | ||
| 13 | Distant Station Connected | Indicator from ACE to DTE that the call is successfully made. | DSC |
| 14-17 | Digit Signal Circuits | Four lines containing a parallel BCD dial digit (10 digits, plus control digits) | NB1-NB8 |
| 18 | unused | ||
| 19 | unused | ||
| 20 | unused | ||
| 21 | unused | ||
| 22 | Data Line Occupied | An indicator that is used by the ACE to let the DTE know that the line it wants to use is used by another device. | DLO |
| 23 | unused | ||
| 24 | unused | ||
| 25 | unused |
Notes
The RS-366 interface uses the same 25-position DB-25 connector and RS-232 electrical signalling levels as EIA-232. The DTE presents a 4-bit BCD digit on lines NB1–NB8 and then asserts Digit Present (DPR); the ACE acknowledges readiness for the next digit via Present Next Digit (PND). Digit code 10 is conventionally used to signal “wait for dial tone” and digit code 11 to signal end-of-number. The interface operates in parallel — digits are clocked one at a time rather than transmitted serially — making it distinct from the Hayes AT-command approach that encodes dial strings as ASCII text on the data channel. The standard was largely made obsolete by smart modems implementing the Hayes command set.
Signals
- DPR (Digit Present, pin 2) — asserted by the DTE to inform the ACE that a valid BCD digit is present on lines NB1–NB8.
- ACR (Abandon Call and Retry, pin 3) — asserted by the ACE to indicate that a connection attempt failed (e.g., busy signal received); the DTE may retry.
- CRQ (Call Request, pin 4) — asserted by the DTE to instruct the ACE to go off-hook and begin a call attempt.
- PND (Present Next Digit, pin 5) — asserted by the ACE to indicate readiness to accept the next BCD digit from the DTE.
- DSC (Distant Station Connected, pin 13) — asserted by the ACE to inform the DTE that the called party has answered and the connection is established.
- NB1–NB8 (Digit Signal Circuits, pins 14–17) — four lines encoding one BCD digit in parallel: NB1 (pin 14, 1-bit weight), NB2 (pin 15, 2-bit weight), NB4 (pin 16, 4-bit weight), NB8 (pin 17, 8-bit weight). Digit 10 signals “wait for dial tone”; digit 11 signals end-of-number.
- DLO (Data Line Occupied, pin 22) — asserted by the ACE to indicate that the telephone line is already in use by another device and cannot be seized.
References
- AN-917: Popular Connector Pin Assignments for Data Communications — Texas Instruments; graphical pin-assignment reference covering RS-366 and related EIA serial standards.
- Fundamentals of RS-232 Serial Communications — Analog Devices; RS-232 electrical signalling overview applicable to RS-366.