Rom - HP PageWriter XL M1705B Service Manual

System Memory
The system memory circuitry consists of the DRAM array, the
DRAM gate array, and two data transceivers, as shown in Figure 5-5.
f-7
The system memory provides storage for ECG data, printer image
construction, signal processing needs, and system software.
The DRAM gate array controls RAM operations, including read,
write, and refresh cycles. The refresh cycle continues when the
cardiograph is in Standby. The DRAM gate array also provides the
following:
w A watchdog timer-shuts off the system after a software selected
delay if not reset by software.
1 A real-time counter (RTC)-keeps track of elapsed time since the
last RTC reset.
n
Power supply support-manages the power-on and power-off event
sequences.
The data transceivers buffer the data on the CPU data bus to
provide adequate drive for the RAM array.
The system memory circuitry remains energized regardless of whether
the cardiograph is On or in Standby. The only times that power is
lost to system memory are when:
w The battery is not installed WKZ the cardiograph is disconnected
from AC power.
w The cardiograph enters the low battery shutdown condition.
When all power is lost to system memory, its stored data is lost
including the configured software. This makes it necessary to reload
the software from disk'.
ROM
The read-only memory consists of a 64Kxl6 EPROM (Ml700A and
M1701A), which stores the code tha.t starts the system. The M1702A
read-only memory consists of a 12Sk x 16 EPROM, which stores the
cardiograph software.
5-10 Theory of Operation