Single-channel RS-232 transceiver that runs the +/-5 V-class RS-232 line levels from a 3 to 5.5 V logic supply, using an on-chip charge pump (the C1/C2/V+/V- capacitor network) so no negative rail is needed. One driver (TTL in to RS-232 out) and one receiver (RS-232 in to TTL out) cover a basic serial port, and an auto-shutdown plus FORCEON/FORCEOFF and INVALID pins drop supply current when the line is idle. A compact 3.3 V-capable RS-232 front end for a single serial channel.

Pinout

Pin	Name	Type	Function
1	EN	input	Active-low receiver enable. Drive low for normal receiver operation; drive high to force R_OUT into high-Z. Logic threshold VIL=0.8V max, VIH=2.0V (3.3V) / 2.4V (5.0V).
2	C1+	passive	Positive terminal of the voltage-doubler charge-pump flying capacitor (C1). Typ 0.1µF at 3.3V, 0.047µF at 5.0V (Note 2).
3	V+	power_out	+5.5V output generated by the doubling charge pump. Bypass to GND with the reservoir capacitor C3 (0.1µF typ at 3.3V, 0.33µF at 5.0V).
4	C1-	passive	Negative terminal of the voltage-doubler charge-pump flying capacitor (C1).
5	C2+	passive	Positive terminal of the inverting charge-pump flying capacitor (C2). Typ 0.1µF at 3.3V, 0.33µF at 5.0V.
6	C2-	passive	Negative terminal of the inverting charge-pump flying capacitor (C2).
7	V-	power_out	-5.5V output generated by the inverting charge pump. Bypass to GND with reservoir capacitor C4.
8	R_IN	input	RS-232 receiver input. Accepts ±25V EIA/TIA-232 levels; typical input resistance 5kΩ (3-7kΩ). VIL ≈ 0.6-1.2V, VIH ≈ 1.5-2.4V, 0.3V typical hysteresis.
9	R_OUT	tri_state	TTL/CMOS receiver output (inverted from R_IN). Three-state: high-Z in shutdown, when EN is high, or when the part is in AutoShutdown. VOL ≤ 0.4V at 1.6mA; VOH ≥ VCC-0.6V at -1.0mA.
10	INVALID	output	Active-low valid-signal detector output. Low (asserted) while all receiver inputs are between -0.3V and +0.3V for ≥30µs (invalid RS-232 present); high when a valid RS-232 level (>+2.7V or <-2.7V) appears. Useful for DTR/RI monitoring.
11	T_OUT	output	RS-232 transmitter output. Inverting driver to EIA/TIA-232 levels, ±5.0V typ swing into 3kΩ‖1000pF, short-circuit current ±35mA typ. High-Z in shutdown.
12	FORCEON	input	Active-high override that keeps the transmitter and charge pumps on regardless of receiver-input activity (FORCEOFF must be high). See AutoShutdown truth table.
13	T_IN	input	TTL/CMOS transmitter input. Drives the inverting RS-232 transmitter. VIL ≤ 0.8V, VIH ≥ 2.0V (3.3V) / 2.4V (5.0V).
14	GND	power_in	Device ground reference for logic and charge-pump return.
15	VCC	power_in	+3.0V to +5.5V supply input. Bypass with 0.1µF to GND. Absolute max -0.3V to +6V.
16	FORCEOFF	input	Active-low master shutdown. Drive low to disable transmitters and on-board power supply (overrides FORCEON and all AutoShutdown circuitry). Tie high for normal operation; can be tied to FORCEON as a single SHDN-style input.

Key specifications

Parameter	Value
Type	RS-232 transceiver (1 driver, 1 receiver)
Supply	3-5.5 V
Line levels	RS-232 via on-chip charge pump
Power control	Auto-shutdown, FORCEON / FORCEOFF
Status	INVALID (valid-signal detect)
Data rate	Up to 250 kbps
Package	SSOP-16

Ordering variants

Same part, different packaging or reel option.

MAX3221CAE (SSOP-16)
MAX3221IPWR (TSSOP-16)
MAX3221EAE (+/-15 kV ESD)

CAD (KiCad official)

Symbol: Interface_UART:MAX3221 (KiCad official)
Footprint: Package_SO:SSOP-16_5.3x6.2mm_P0.65mm
3D model: SSOP-16 (KiCad packages3D, STEP)

Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

Datasheet: Analog Devices datasheet (16 pp, sha256 4b10ea08c7155060, retrieved 2026-08-17, tier: manufacturer)
Extractor: ds2sf (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
CAD: KiCad official libraries (CC-BY-SA 4.0).

Datasheet PDF