MC6845 CRT controller pinout

DIP-40. Makes sync, display enable, cursor and the video RAM refresh addresses.

Counts characters and scan lines from a character clock and gives the video RAM address (MA0 to MA13), the scan line in the row (RA0 to RA4), HS, VS, display enable and cursor. Registers are written through an index at $3B4 and data at $3B5 on the MDA, and $3D4 and $3D5 on the CGA and PCjr. On the IBM cards the character clock comes from the card dot counter; HS runs at about 18 kHz on the MDA and 15.7 kHz on the CGA. HS is undefined until register R2 is programmed, so a dead or unprogrammed 6845 gives no sync. Not pin-identical to the 6545.

Open the interactive 6845 pinout

Pins

Pin	Name	Type	Signal
1	GND	ground	Ground (VSS).
2	/RESET	ctrl	Reset, active LOW, with LPSTB LOW (RESET LOW with LPSTB HIGH selects test mode). Clears the counters, stops the display and drives the outputs LOW; HS is not defined until R2 is programmed. The control registers keep their values. Hold it LOW for at least one character clock.
3	LPSTB	video	Light pen strobe. A rising edge latches the current refresh address into the light pen register.
4	MA0	addr	Refresh memory address 0. Scans the video RAM and keeps running in retrace.
5	MA1	addr	Refresh memory address 1. Scans the video RAM and keeps running in retrace.
6	MA2	addr	Refresh memory address 2. Scans the video RAM and keeps running in retrace.
7	MA3	addr	Refresh memory address 3. Scans the video RAM and keeps running in retrace.
8	MA4	addr	Refresh memory address 4. Scans the video RAM and keeps running in retrace.
9	MA5	addr	Refresh memory address 5. Scans the video RAM and keeps running in retrace.
10	MA6	addr	Refresh memory address 6. Scans the video RAM and keeps running in retrace.
11	MA7	addr	Refresh memory address 7. Scans the video RAM and keeps running in retrace.
12	MA8	addr	Refresh memory address 8. Scans the video RAM and keeps running in retrace.
13	MA9	addr	Refresh memory address 9. Scans the video RAM and keeps running in retrace.
14	MA10	addr	Refresh memory address 10. Scans the video RAM and keeps running in retrace.
15	MA11	addr	Refresh memory address 11. Scans the video RAM and keeps running in retrace.
16	MA12	addr	Refresh memory address 12. Scans the video RAM and keeps running in retrace.
17	MA13	addr	Refresh memory address 13. Scans the video RAM and keeps running in retrace.
18	DE	video	Display enable, active HIGH, during the visible area.
19	CURSOR	video	Cursor, active HIGH, at the programmed cursor address.
20	VCC	power	+5 V supply, 5 percent.
21	CLK	clock	Character clock input. Active edge HIGH to LOW.
22	R/W	ctrl	Read (HIGH) or write (LOW) of the register file.
23	E	ctrl	Enable. Clocks data in and out of the data bus; active edge HIGH to LOW.
24	RS	ctrl	Register select. LOW: the index (address) register. HIGH: the register it points to.
25	/CS	ctrl	Chip select, active LOW.
26	D7	data	Data bus bit 7.
27	D6	data	Data bus bit 6.
28	D5	data	Data bus bit 5.
29	D4	data	Data bus bit 4.
30	D3	data	Data bus bit 3.
31	D2	data	Data bus bit 2.
32	D1	data	Data bus bit 1.
33	D0	data	Data bus bit 0.
34	RA4	addr	Row address 4: the scan line within the character row, to the character generator.
35	RA3	addr	Row address 3.
36	RA2	addr	Row address 2.
37	RA1	addr	Row address 1.
38	RA0	addr	Row address 0.
39	HS	video	Horizontal sync, active HIGH, TTL.
40	VS	video	Vertical sync, active HIGH, TTL.

Fitted in

IBM PC 5150 MDA/CGA card (not the planar)
IBM XT 5160
IBM Portable 5155
IBM PCjr 4860 ZM21
Amstrad CPC 464 IC108 (HD6845SP)
Amstrad CPC 664 IC108 (HD6845SP)
Amstrad CPC 6128 IC108 (HD6845SP)
BBC Micro Model B IC2
BBC Master 128 IC22

MC6845 CRT controller pinout

Pins

Fitted in

Sources