the3dfxdude
Experienced Member
Hi,
I am testing a QuadRam card with a 8250 UART serial port that I previously could not use due to no data being exchanged in programs. The loopback testing information and program used is thanks to modem7 in a recent posting:
Having this port working is nice so I do not have to swap cards to have a working serial port in my 5150 when the quadram card should do. The internal loopback tests all pass.
For the external loopback tests, the TxD/RxD test fails, but all other pins pass. Tracing the pin 2/3 on the port, I found line driver XR1488N and line receiver XR1489AN which are the level converters here for RS232, which I suppose are compatible with MC1488 and MC1489A.
I remembered probing pin 2 in the past not seeing anything come across on a scope, I decided today to try the external loopback test program and my scope on a working card. In that test, I see that for the working card, that the output from pin 2 drives a low voltage level (In this case, a low voltage means a negative voltage level, not zero. I didn't check the exact level.), and high voltage level pulses when the test sends data. It is otherwise low when idle. So I checked the bad port one more time, now knowing how it should look during the test. When testing the quadram card, the TxD pin appears to be driving the output always at a high level, never switching. This means the driver is stuck outputting high, and a bad driver. Is this a good enough conclusion?
In this loopback test, I appear to have a bad 1488 driver. This current situation does not appear to tell me if the 1489A chip is ok for the RxD pin. I do remember when using the port previously, that I was able to see some data going in one direction, but that was with a crudely written program where I was trying to identify the culprit.
I will be ordering some chips to solve this eventually since I don't have these laying around. Hopefully I do not also need to buy the receiver chip. Do you have any experiences to share on which of these chips go bad usually?
I am testing a QuadRam card with a 8250 UART serial port that I previously could not use due to no data being exchanged in programs. The loopback testing information and program used is thanks to modem7 in a recent posting:
Having this port working is nice so I do not have to swap cards to have a working serial port in my 5150 when the quadram card should do. The internal loopback tests all pass.
For the external loopback tests, the TxD/RxD test fails, but all other pins pass. Tracing the pin 2/3 on the port, I found line driver XR1488N and line receiver XR1489AN which are the level converters here for RS232, which I suppose are compatible with MC1488 and MC1489A.
I remembered probing pin 2 in the past not seeing anything come across on a scope, I decided today to try the external loopback test program and my scope on a working card. In that test, I see that for the working card, that the output from pin 2 drives a low voltage level (In this case, a low voltage means a negative voltage level, not zero. I didn't check the exact level.), and high voltage level pulses when the test sends data. It is otherwise low when idle. So I checked the bad port one more time, now knowing how it should look during the test. When testing the quadram card, the TxD pin appears to be driving the output always at a high level, never switching. This means the driver is stuck outputting high, and a bad driver. Is this a good enough conclusion?
In this loopback test, I appear to have a bad 1488 driver. This current situation does not appear to tell me if the 1489A chip is ok for the RxD pin. I do remember when using the port previously, that I was able to see some data going in one direction, but that was with a crudely written program where I was trying to identify the culprit.
I will be ordering some chips to solve this eventually since I don't have these laying around. Hopefully I do not also need to buy the receiver chip. Do you have any experiences to share on which of these chips go bad usually?